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MAN'S PLACE IN THE UNIVERSE 



O, glittering host ! O, golden line ! 

I would I had an angel's ken, 
Your deepest secrets to divine, 

And read your mysteries to men." 



MANS PLACE IN THE UNIVERSE 

A STUDY OF THE RESULTS OF SCIEN- 
TIFIC RESEARCH IN RELATION TO THE 
UNITY OR PLURALITY OF WORLDS 



BY 



ALFRED R. WALLACE, LL.D., D.C.L., F.R.S. 




NEW YORK 

McCLURE, PHILLIPS # CO. 

MCMIII 






7 fi» 1a if. 



L„l 



Copyright, 1903, by 
McCLURE, PHILLIPS & CO. 









Published, November, 1903, N 



I said unto my inmost heart 

Shall I don corslet, helm, and shield, 
And shall I with a Giant strive, 

And charge a Dragon on the field ? " 

J. H. Dell. 



PREFACE 

This work has been written in consequence of the great interest 
excited by my article, under the same title, which appeared simul- 
taneously in The Fortnightly Review and the New York Inde- 
pendent. Two friends who read the manuscript were of opinion 
that a volume, in which the evidence could be given much more 
fully, would be desirable, and the result of the publication of the 
article confirmed their view. 

I was led to a study of the subject when writing four new chap- 
ters on Astronomy for a new edition of The Wonderful Century. 
I then found that almost all writers on general astronomy, from 
Sir John Herschell to Professor Simon Newcomb and Sir Norman 
Lockyer, stated, as an indisputable fact, that our sun is situated 
in the plane of the great ring of the Milky Way, and also very 
nearly in the centre of that ring. The most recent researches 
also showed that there was little or no proof of there being any 
stars or nebulae very far beyond the Milky Way, which thus 
seemed to be the limit, in that direction, of the stellar universe. 

Turning to the earth and the other planets of the Solar Sys- 
tem, I found that the most recent researches led to the conclusion 
that no other planet was likely to be the seat of organic life, 
unless perhaps of a very low type. For many years I had paid spe- 
cial attention to the problem of the measurement of geological time, 
and also that of the mild climates and generally uniform condi- 
tions that had prevailed throughout all geological epochs ; and on 



vi PREFACE 

considering the number of concurrent causes and the delicate bal- 
ance of conditions required to maintain such uniformity, I became 
still more convinced that the evidence was exceedingly strong 
against the probability or possibility of any other planet being 
inhabited. 

Having long been acquainted with most of the works dealing 
with the question of the supposed Plurality of Worlds, I was quite 
aware of the very superficial treatment the subject had received, 
even in the hands of the most able writers, and this made me the 
more willing to set forth the whole of the available evidence — 
astronomical, physical, and biological — in such a way as to show 
both what was proved and what suggested by it. 

The present work is the result, and I venture to think that 
those who will read it carefully will admit that it is a book that 
was worth writing. It is founded almost entirely on the marvel- 
lous body of facts and conclusions of the New Astronomy together 
with those reached by modern physicists, chemists, and biologists. 
Its novelty consists in combining the various results of these dif- 
ferent branches of science into a connected whole, so as to show 
their bearing upon a single problem — a problem which is of very 
great interest to ourselves. 

This problem is, whether or no the logical inferences to be 
drawn from the various results of modern science, lend support to 
the view that our earth is the only inhabited planet, not only in 
the Solar System but in the whole stellar universe. Of course it 
is a point as to which absolute demonstration, one way or the 
other, is impossible. But in the absence of any direct proofs, it 
is clearly rational to inquire into probabilities ; and these proba- 
bilities must be determined not by our prepossessions for any par- 
ticular view, but by an absolutely impartial and unprejudiced 
examination of the tendency of the evidence. 



PREFACE vii 

As the book is written for the general, educated body of read- 
ers, many of whom may not be acquainted with any aspect of the 
subject or with the wonderful advance of recent knowledge in that 
department often termed the New Astronomy, a popular account 
has been given of all those branches of it which bear upon the 
special subject here discussed. This part of the work occupies the 
first six chapters. Those who are fairly acquainted with modern 
astronomical literature, as given in popular works, may begin at 
my seventh chapter, which marks the commencement of the con- 
siderable body of evidence and of argument I have been able to 
adduce. 

To those of my readers who may have been influenced by any 
of the adverse criticisms on my views as set forth in the article 
already referred to, I must again urge, that throughout the whole 
of this work, neither the facts nor the more obvious conclusions 
from the facts are given on my own authority, but always on that 
of the best astronomers, mathematicians, and other men of science 
to whose works I have had access, and whose names, with exact 
references, I generally give. 

What I claim to have done is, to have brought together the 
various facts and phenomena they have accumulated ; to have set 
forth the hypotheses by which they account for them, or the results 
to which the evidence clearly points ; to have judged between 
conflicting opinions and theories ; and lastly, to have combined 
the results of the various widely-separated departments of science, 
and to have shown how they bear upon the great problem which 
I have here endeavoured, in some slight degree, to elucidate. 

As such a large body of facts and arguments from distinct sci- 
ences have been here brought together, I have given a rather full 
summary of the whole argument, and have stated my final conclu- 
sions in six short sentences. I then briefly discuss the two aspects 



viii PREFACE 

of the whole problem — those from the materialistic and from the 
spiritualistic points of view; and I conclude with a few general 
observations on the almost unthinkable problems raised by ideas 
of Infinity — problems which some of my critics thought I had 
attempted in some degree to deal with, but which, I here point 
out, are altogether above and beyond the questions I have dis- 
cussed, and equally above and beyond the highest powers of the 
human intellect. 

Broadstone, Dorset, 
September 1903. 



The wilder'd mind is tost and lost, 

O sea in thy eternal tide ; 
The reeling brain essays in vain, 

O stars, to grasp the vastness wide ! 
The terrible tremendous scheme 

That glimmers in each glancing light, 
O night, O stars, too rudely jars 

The finite with the infinite ! " 

J. H. Deli, 



CONTENTS 

CHAPTER PAKE 

I. Early Ideas as to the Universe and Its Relation to Man, 3 

II. Modern Ideas as to Man's Relation to the Universe, . 9 

III. The New Astronomy, 25 

IV. The Distribution of the Stars, 47 

V. Distance of the Stars — The Sun's Motion through Space, 73 

VI. The Unity and Evolution of the Star System, . . 99 

VII. Are the Stars Infinite in Number? 135 

VIII. Our Relation to the Milky Way, 156 

IX. The Uniformity of Matter and Its Laws throughout 

the Stellar Universe, 182 

X. The Essential Characters of the Living Organism, . . 190 
XI. The Physical Conditions Essential for Organic Life, . 205 
XII. The Earth in Its Relation to the Development and Main- 
tenance of Life, 216 

XIII. The Earth in Relation to Life: Atmospheric Conditions, 240 

XIV. The Earth is the Only Habitable Planet in the Solar 

System, 258 

XV. The Stars — Have They Planetary Systems? Are They 

Beneficial to Us? 278 

1 

XVI. Stability of the Star-Systems: Importance of Our Central 

Position: Summary and Conclusion, .... 291 



" Who is man, and what his place ? 
Anxious asks the heart, perplext 
In this recklessness of space, 

Worlds with worlds thus intermixt : 
What has he, this atom creature, 
In the infinitude of Nature ? " 

F. T. Palgrave. 



MAN'S PLACE IN THE UNIVERSE 



CHAPTER I 

EARLY IDEAS AS TO THE UNIVERSE AND ITS RELATION TO MAN 

When men attained to sufficient intelligence for speculations 
as to their own nature and that of the earth on which they lived, 
they must have been profoundly impressed by the nightly pa- 
geant of the starry heavens. The intense sparkling brilliancy of 
Sirius and Vega, the more massive and steady luminosity of 
Jupiter and Venus, the strange grouping of the brighter stars 
into constellations to which fantastic names indicating their 
resemblance to various animals or terrestrial objects seemed ap- 
propriate and were soon generally adopted, together with the ap- 
parently innumerable stars of less and less brilliancy scattered 
broadcast over the sky, many only being visible on the clearest 
nights and to the acutest vision, constituted altogether a scene of 
marvellous and impressive splendour of which it must have seemed 
almost impossible to attain any real knowledge, but which 
afforded an endless field for the imagination of the observer. 

The relation of the stars to the sun and moon in their respective 
motions was one of the earliest problems for the astronomer, and 
it was only solved by careful and continuous observation, which 
showed that the invisibility of the former during the day was 
wholly due to the blaze of light, and this is said to have been 
proved at an early period by the observed fact that from the 



4 MAN'S PLACE IN THE UNIVERSE 

bottom of very deep wells stars can be seen while the sun is shin- 
ing. During total eclipses of the sun also the brighter stars be- 
come visible, and, taken in connection with the fixity of position 
of the pole-star, and the course of those circumpolar stars which 
never set in the latitudes of Greece, Egypt, and Chaldea, it soon 
became possible to frame a simple hypothesis which supposed the 
earth to be suspended in space, while at an unknown distance from 
it a crystal sphere revolved upon an axis indicated by the pole- 
star, and carried with it the whole host of heavenly bodies. This 
was the theory of Anaximander (540 b. c), and it served as the 
starting-point for the more complex theory which continued to 
be held in various forms and with endless modifications down to 
the end of the sixteenth century. 

It is believed that the early Greeks obtained some knowledge of 
astronomy from the Chaldeans, who appear to have been the first 
systematic observers of the heavenly bodies by means of instru- 
ments, and who are said to have discovered the cycle of eighteen 
years and ten days after which the sun and moon return to the 
same relative positions as seen from the earth. The Egyptians 
perhaps derived their knowledge from the same source, but there 
is no proof that they were great observers, and the accurate 
orientation, proportions, and angles of the Great Pyramid and 
its inner passages may perhaps indicate a Chaldean architect. 

The very obvious dependence of the whole life of the earth 
upon the sun, as a giver of heat and light, sufficiently explains 
the origin of the belief that the latter was a mere appanage of 
the former ; and as the moon also illuminates the night, while the 
stars as a whole also give a very perceptible amount of light, 



EARLY IDEAS 5 

especially in the dry climate and clear atmosphere of the East, 
and when compared with the pitchy darkness of cloudy nights 
when the moon is below the horizon, it seemed clear that the whole 
of these grand luminaries — sun, moon, stars, and planets — were 
but parts of the terrestrial system, and existed solely for the 
benefit of its inhabitants. 

Empedocles (444 b. c. ) is said to have been the first who 
separated the planets from the fixed stars, by observing their 
very peculiar motions, while Pythagoras and his followers deter- 
mined correctly the order of their succession from Mercury to 
Saturn. No attempt was made to explain these motions till a 
century later, when Eudoxus of Cnidos, a contemporary of Plato 
and of Aristotle, resided for some time in Egypt, where he be- 
came a skilful astronomer. He was the first who systematically 
worked out and explained the various motions of the heavenly 
bodies on the theory of circular and uniform motion round the 
earth as a centre, by means of a series of concentric spheres, 
each revolving at a different rate and on a different axis, but so 
united that all shared in the motion round the polar axis. The 
moon, for example, was supposed to be carried by three spheres ; 
the first revolved parallel to the equator and accounted for the 
diurnal motion — the rising and setting — of the moon, another 
moved parallel to the ecliptic and explained the monthly changes 
of the moon, while the third revolved at the same rate but more 
obliquely, and explained the inclination of the moon's orbit to 
that of the earth. In the same way, each of the five planets had 
four spheres, two moving like the first two of the moon, another 
one also moving in the ecliptic was required to explain the retro- 



6 MAN'S PLACE IN THE UNIVERSE 

grade motion of the planets, while a fourth oblique to the ecliptic 
was needed to explain the diverging motions due to the different 
obliquity of the orbit of each planet to that of the earth. This 
was the celebrated Ptolemaic system in the simplest form needed 
to account for the more obvious motions of the heavenly bodies. 
But in the course of ages the Greek and Arabian astronomical 
observers discovered small divergencies due to the various degrees 
of excentricity of the orbits of the moon and planets and their 
consequent varying rates of motion ; and to explain these other 
spheres were added, together with smaller circles sometimes re- 
volving excentrically, so that at length about sixty of these 
spheres, epicycles, and excentrics were required to account for the 
various motions observed with the rude instruments, and the rates 
of motion determined by the very imperfect time-measurers of 
those early ages. And although a few great philosophers had 
at different times rejected this cumbrous system and had en- 
deavoured to promulgate more correct ideas, their views had no 
influence on public opinion even among astronomers and mathe- 
maticians, and the Ptolemaic system held full sway down to the 
time of Copernicus, and was not finally given up till Kepler's 
Laws and Galileo's Dialogues compelled the adoption of simpler 
and more intelligible theories. 

We are now so accustomed to look upon the main facts of 
astronomy as mere elementary knowledge that it is difficult for 
us to picture to ourselves the state of almost complete ignorance 
which prevailed even among the most civilised nations throughout 
antiquity and the Middle Ages. The rotundity of the earth was 
held by a few at a very early period, and was fairly well estab- 



EARLY IDEAS 7 

lished in later classical times. The rough determination of the 
size of our globe followed soon after ; and when instrumental ob- 
servations became more perfect, the distance and size of the moon 
were measured with sufficient accuracy to show that it was very 
much smaller than the earth. But this was the furthest limit 
of the determination of astronomical sizes and distances before the 
discovery of the telescope. Of the sun's real distance and size 
nothing was known except that it was much farther from us and 
much larger than the moon ; but even in the century before the 
commencement of the Christian era Posidonius determined the 
circumference of the earth to be 240,000 stadia, equal to about 
28,600 miles, a wonderfully close approximation considering the 
very imperfect data at his command. He is also said to have 
calculated the sun's distance, making it only one-third less than 
the true amount, but this must have been a chance coincidence, 
since he had no means of measuring angles more accurately than 
to one degree, whereas in the determination of the sun's distance 
instruments are required which measure to a second of arc. 

Before the discovery of the telescope the sizes of the planets 
were quite unknown, while the most that could be ascertained 
about the stars was, that they were at a very great distance from 
us. This being the extent of the knowledge of the ancients as 
to the actual dimensions and constitution of the visible universe, 
of which, be it remembered, the earth was held to be the centre, we 
cannot be surprised at the almost universal belief that this uni- 
verse existed solely for the earth and its inhabitants. In classical 
times it was held to be at once the dwelling-place of the gods and 
their gift to man, while in Christian ages this belief was but 



8 MAN'S PLACE IN THE UNIVERSE 

slightly, if at all, changed ; and in both it would have been con- 
sidered impious to maintain that the planets and stars did not 
exist for the service and delight of mankind alone but in all prob- 
ability had their own inhabitants, who might in some cases be 
even superior in intellect to man himself. But apparently, dur- 
ing the whole period of which we are now treating, no one was 
so daring as even to suggest that there were other worlds with 
other inhabitants, and it was no doubt because the idea that we 
occupied the world, the very centre of the whole surrounding 
universe which existed solely for us, that the discoveries of 
Copernicus, Tycho Brahe, Kepler, and Galileo excited so much 
antagonism and were held to be impious and altogether incredible. 
They seemed to upset the whole accepted order of nature, and to 
degrade man by removing his dwelling-place, the earth, from 
the commanding central position it had always before occupied. 



CHAPTER II 

MODERN IDEAS AS TO MAN'S RELATION TO THE UNIVERSE 

The beliefs as to the subordinate position held by sun, moon, 
and stars in relation to the earth, which were almost universal 
down to the time of Copernicus, began to give way when the dis- 
coveries of Kepler, and the revelations of the telescope, demon- 
strated that our earth was not specially distinguished from the 
other planets by any superiority of size or position. The idea 
at once arose that the other planets might be inhabited ; and when 
the rapidly increasing power of the telescope, and of astronomical 
instruments generally, revealed the wonders of the solar system 
and the ever-increasing numbers of the fixed stars, the belief 
in other inhabited worlds became as general as the opposite 
belief had been in all preceding ages, and it is still held in 
modified forms to the present day. 

But it may be truly said that the later like the earlier belief 
is founded more upon religious ideas than upon a scientific and 
careful examination of the whole of the facts, astronomical, 
physical, and biological, and we must agree with the late Dr. 
Whewell, that the belief that other planets are inhabited has been 
generally entertained, not in consequence of physical reasons but 
in spite of them. And he adds : " It was held that Venus, or 
that Saturn was inhabited, not because anyone could devise, 

9 



10 MAN'S PLACE IN THE UNIVERSE 

with any degree of probability, any organised structure which 
would be suitable to animal existence on the surfaces of those 
planets ; but because it was conceived that the greatness or good- 
ness of the Creator, or His wisdom, or some other of His attri- 
butes, would be manifestly imperfect, if these planets were not 
tenanted by living creatures." Those persons who have only 
heard that many eminent astronomers down to our own day have 
upheld the belief in a Plurality of Worlds will naturally suppose 
that there must be some very cogent arguments in its favour, 
and that it must be supported by a considerable body of more 
or less conclusive facts. They will therefore probably be sur- 
prised to hear that any direct evidence which may be held to 
support the view is almost wholly wanting, and that the greater 
part of the arguments are weak and flimsy in the extreme. 

Of late years, it is true, some few writers have ventured to 
point out how many difficulties there are in the way of accepting 
the belief , but even these have never examined the question from 
the various points of view which are essential to a proper con- 
sideration of it ; while, so far as it is still upheld, it is thought 
sufficient to show, that in the case of some of the planets, there 
seem to be such conditions as to render life possible. In the 
millions of planetary systems supposed to exist it is held to be 
incredible that there are not great numbers as well fitted to be 
inhabited by animals of all grades, including some as high as 
man or even higher, and that we must, therefore, believe that 
they are so inhabited. As in the present work I propose to show, 
that the probabilities and the weight of direct evidence tend to 
an exactly opposite conclusion, it will be well to pass briefly in 



MODERN IDEAS 11 

review the various writers on the subj ect, and to give some indica- 
tion of the arguments they have used and the facts they have set 
forth. For the earlier upholders of the theory I am indebted 
to Dr. Whewell, who, in his Dialogue on the Plurality of Worlds 
— a Supplement to his well-known volume on the subject — refers 
to all writers of importance known to him. 

The earliest are the great astronomers Kepler and Huygens, 
and the learned Bishop Wilkins, who all believed that the moon 
was or might probably be inhabited ; and of these Whewell con- 
siders Wilkins to have been by far the most thoughtful and 
earnest in supporting his views. Then we have Sir Isaac Newton 
himself, who, at considerable length, argued that the sun was 
probably inhabited. But the first regular work devoted to the 
subject appears to have been written by M. Fontenelle, Secretary 
to the Academy of Sciences in Paris, who in 1686 published his 
Conversations on the Plurality of Worlds. The book consisted 
of five chapters, the first explaining the Copernican Theory ; the 
second maintaining that the moon is a habitable world ; the third 
gives particulars as to the moon, and argues that the other planets 
are also inhabited ; the fourth gives details as to the worlds of the 
five planets ; while the fifth declares that the fixed stars are suns, 
and that each illuminates a world. This work was so well written, 
and the subject proved so attractive, that it was translated into 
all the chief European languages, while the astronomer Lalande 
edited one of the French editions. Three English translations 
were published, and one of these went through six editions down 
to the year 1737. The influence of this work was very great and 
no doubt led to that general acceptance of the theory by such 



12 MAN'S PLACE IN THE UNIVERSE 

men as Sir William Herschel, Sir John Herschel, Dr. Chalmers, 
Dr. Dick, Dr. Isaac Taylor, and M. Arago, although it was 
wholly founded on pure speculation, and there was nothing that 
could be called evidence on one side or the other. 

This was the state of public opinion when an anonymous work 
appeared (in 1853) under the somewhat misleading title of The 
Plurality of Worlds: An Essay, This was written, as already 
stated, by Dr. Whewell, who, for the first time, ventured to doubt 
the generally accepted theory, and showed that all the evidence at 
our command led to the conclusion that some of the planets were 
certainly not habitable, that others were probably not so, while in 
none was there that close correspondence with terrestrial condi- 
tions which seemed essential for their habitability by the higher 
animals or by man. The book was ably written and showed 
considerable knowledge of the science of the time, but it was very 
diffuse, and the larger part of it was devoted to showing that his 
views were not in any way opposed to religion. One of his best 
arguments was founded on the proposition that The Earth's 
Orbit is the Temperate Zone of the Solar System, that there only 
is it possible to have those moderate variations of heat and cold, 
dryness and moisture, which are suitable for animal life. He 
v suggested that the outer planets of the system consisted mainly 
of water, gases, and vapour, as indicated by their low specific 
gravity, and were therefore quite unsuitable for terrestrial life; 
while those near the sun were equally unsuited, because owing 
to the great amount of solar heat water could not exist on their 
surfaces. He devotes a great deal of space to the evidence that 
there is no animal life on the moon, and taking this as proved, he 



MODERN IDEAS 13 

uses it as a counter argument against the other side. They 
always urge that, the earth being inhabited, we must suppose 
the other planets to be so too ; to which he replies : We know that 
the moon is not inhabited though it has all the advantage of 
proximity to the sun that the earth has; why then should not 
other planets be equally uninhabited? 

He then comes to Mars and admits that this planet is very 
like the earth so far as we can judge, and that it may therefore 
be inhabited, or as the author expresses it, " may have been 
judged worthy of inhabitants by its Maker." But he urges the 
small size of Mars, its coldness owing to distance from the sun, 
and that the annual melting of its polar ice-caps will keep it cold 
all through the summer. If there are animals they are probably 
of a low type like the saurians and iguanodons of our seas 
during the Weal den epoch ; but, he argues, as even on our earth 
the long process of preparation for man was carried on for count- 
less millions of years, we need not discuss whether there are intelli- 
gent beings on Mars till we have some better evidence that there 
are any living creatures at all. 

Several of the early chapters are devoted to an attempt to 
minimise the difficulties of those religious persons who feel op- 
pressed by the immensity and complexity of the material universe 
as revealed by modern astronomy; and by the almost infinite 
insignificance of man and his dwelling-place, the earth, in com- 
parison with it, an insignificance vastly increased if not only the 
planets of the solar system, but also those which circle around 
the myriads of suns, are also theatres of life. And these persons 
are further disquieted because the very same facts are used by 



14 MAN'S PLACE IN THE UNIVERSE 

sceptics of various kinds in their attacks upon Christianity. 
Such writers point out the irrationality and absurdity of sup- 
posing that the Creator of all this unimaginable vastness of suns 
and systems, filling for all we know endless space, should take 
any special interest in so mean and pitiful a creature as man, 
the imperfectly developed inhabitant of one of the smaller worlds 
attached to a second- or third-rate sun, a being whose whole his- 
tory is one of war and bloodshed, of tyranny, torture, and death ; 
whose awful record is pictured by himself in such books as 
Josephus' History of the Jews, the Decline and Fall of the 
Roman Empire, and even more forcibly summarised in that 
terrible picture of human fiendishness and misery, The Martyr- 
dom of Man; while their character is indicated by one of the 
kindest and simplest of their poets in the restrained but express- 
ive lines : 

"Man's inhumanity to Man 
Makes countless thousands mourn." 

Is it for such a being as this, they say, that God should have 
specially revealed His will some thousands of years ago, and find- 
ing that His commands were not obeyed, His will not fulfilled, yet 
ordained for their benefit the necessarily unique sacrifice of His 
Son, in order to save a small portion of these " miserable sinners M 
from the natural and well-deserved consequence of their stupen- 
dous follies, their unimaginable crimes? Such a belief they main- 
tain is too absurd, too incredible, to be held by any rational being, 
and it becomes even less credible and less rational if we maintain 
that there are countless other inhabited worlds. 

It is very difficult for the religious man to make any adequate 



MODERN IDEAS 15 

reply to such an attack as this, and as a result many have felt 
their position to be untenable and have accordingly lost all faith 
in the special dogmas of orthodox Christianity. They feel them- 
selves really to be between the horns of a dilemma. If there are 
myriads of other worlds, it seems incredible that they should each 
be the objects of a special revelation and a special sacrifice. If, 
on the other hand, we are the only intelligent beings that exist 
in the material universe, and are really the highest creative prod- 
uct of a Being of infinite wisdom and power, they cannot but 
wonder at the vast apparent disproportion between the Creator 
and the created, and are sometimes driven to Atheism from the 
hopelessness of comprehending so mean and petty a result as the 
sole outcome of infinite power. 

Whewell tells us that the great preacher, Dr. Chalmers, in his 
Astronomical Discourses, attempted a reply to these difficulties, 
but, in his opinion, not a very successful one ; and a large part of 
his own work is devoted to the same purpose. His main point 
seems to be that we know too little of the universe to arrive at any 
definite conclusions on the question at issue, and that any ideas 
that we may have as to the purposes of the Creator in forming 
the vast system we see around us, are almost sure to be erroneous. 
We must, therefore, be content to remain ignorant, and must 
rest satisfied in the belief that the Creator had a purpose although 
we are not yet permitted to know what it was. And to those who 
urge that in other worlds there may be other laws of nature 
which may render them quite as habitable by intelligent beings 
as our world is for us, he replies, that if we are to suppose new 
laws of nature in order to render each planet habitable, there is 



16 MAN'S PLACE IN THE UNIVERSE 

an end of all rational enquiry on the subject, and we may main- 
tain and believe that animals may live on the moon without air or 
water, and on the sun exposed to heat which vaporises earths and 
metals. 

His concluding argument, and perhaps one of his strongest, 
is that founded upon the dignity of man, as conferring a pre- 
eminence upon the planet which has produced him. " If," he 
says, " man be not merely capable of Virtue and Duty, of uni- 
versal Love and Self -Devotion, but be also immortal ; if his being 
be of infinite duration, his soul created never to die ; then, indeed, 
we may well say that one soul outweighs the whole unintelligent 
creation." And then, addressing the religious world, he urges 
that, if, as they believe, God has redeemed man by the sacrifice 
of His Son, and has given to him a revelation of His will, then 
indeed no other conception is possible than that he is the sole and 
highest product of the universe. " The elevation of millions of 
intellectual, moral, religious, spiritual creatures, to a destiny so 
prepared, consummated, and developed, is no unworthy occupa- 
tion of all the capacities of space, time, and matter." Then 
with a chapter on " The Unity of the World," and one on " The 
Future," neither of which contain anything which adds to the 
force of his argument, the book ends. 

The publication of this able if rather vague and diffuse work, 
contesting popular opinions, was followed by a burst of indignant 
criticism on the part of a man of considerable eminence in some 
branches of physics — Sir David Brewster, but who was very in- 
ferior, both in general knowledge of science and in literary skill, 
to the writer whose views he opposed. The purport of the book 



MODERN IDEAS 17 

in which he set forth his objections is indicated by its title — 
More Worlds than One, the Creed of the Philosopher and the 
Hope of the Christian. Though written with much force and 
conviction it appeals mainly to religious prejudices, and assumes 
throughout that every planet and star is a special creation, and 
that the peculiarities of each were designed for some special 
purpose. " If," he says, " the moOn had been destined to be 
merely a lamp to our earth, there was no occasion to variegate 
its surface with lofty mountains and extinct volcanoes, and 
cover it with large patches of matter that reflect different quan- 
tities of light and give its surface the appearance of continents 
and seas. It would have been a better lamp had it been a smooth 
piece of lime or of chalk." It is, therefore, he thinks, prepared 
for inhabitants ; and then he argues that all the other satellites 
are also inhabited. Again he says that " when it was found that 
Venus was about the same size as the Earth, with mountains and 
valleys, days and nights, and years analogous to our own, the 
absurdity of believing that she had no inhabitants, when no other 
rational purpose could be assigned for her creation, became an 
argument of a certain amount that she was, like the Earth, the 
seat of animal and vegetable life." Then, when it was found 
that Jupiter was so gigantic " as to require four moons to give 
him light, the argument from analogy that he was inhabited be- 
came stronger also, because it extended to two planets." And 
thus each successive planet having certain points of analogy with 
the others becomes an additional argument; so that when we 
take account of all the planets, with atmosphere, and clouds, and 
arctic snows, and trade-winds, the argument from analogy be- 



18 MAN'S PLACE IN THE UNIVERSE 

comes, lie urges, very powerful ; " and the absurdity of the 
opposite opinion, that planets should have moons and no in- 
habitants, atmospheres with no creatures to breathe in them, and 
currents of air without life to be fanned, became a formidable 
argument which few minds, if any, could resist." 

The work is full of such weak and fallacious rhetoric and 
even, if possible, still weaker. Thus after describing double 
stars, he adds : " But no person can believe that two suns could 
be placed in the heavens for no other purpose than to revolve 
round their common centre of gravity " ; and he concludes his 
chapter on the stars thus : " Wherever there is matter there must 
be Life; Life Physical to enjoy its beauties — Life Moral to 
worship its Maker, and Life Intellectual to proclaim His wisdom 
and His power." And again, " A house without tenants, a city 
without citizens, presents to our minds the same idea as a planet 
without life, and a universe without inhabitants. Why the 
house was built, why the city was founded, why the planet was 
made, and why the universe was created, it would be difficult even 
to conjecture." 

Arguments of this kind, which in almost every case beg 
the question at issue, are repeated ad nauseam. But he also 
appeals to the Old Testament to support his views, by quot- 
ing the fine passage in the Psalms, "When I consider Thy 
heavens, the work of Thy fingers ; the moon and the stars which 
Thou hast ordained ; what is man that Thou are mindful of him ? " 
on which he remarks : " We cannot doubt that inspiration revealed 
to him [David] the magnitude, the distances, and the final cause 
of the glorious spheres which fixed his admiration." And after 



MODERN IDEAS 19 

quoting various other passages from the prophets, all as he thinks 
supporting the same view, he sets forth the extraordinary idea as 
a confirmatory argument, that the planets or some of them are 
to be the future abode of man. For, he says, " Man in his 
future state of existence is to consist, as at present, of a spiritual 
nature residing in a corporeal frame. He must live, therefore, 
upon a material planet, subject to all the laws of matter." And 
he concludes thus : " If there is not room, then, on our globe for 
the millions of millions of beings who have lived and died on its 
surface, we can scarcely doubt that their future abode must be 
on some of the primary or secondary planets of the solar system, 
whose inhabitants have ceased to exist, or upon planets which 
have long been in a state of preparation, as our earth was, for 
the advent of intellectual life." 

It is pleasant to turn from such weak and trivial arguments 
to the only other modern works which deal at some length with 
this subject, the late Richard A. Proctor's Other Worlds than 
Ours, and a volume published five years later under the title, 
Our Place among Infinities. Written as these were by one 
of the most accomplished astronomers of his day, remarkable 
alike for the acuteness of his reasoning and the clearness of his 
style, we are always interested and instructed even when we can- 
not agree with his conclusions. In the first work mentioned 
above, he assumes, like Sir David Brewster, the antecedent 
probability that the planets are inhabited and on much the same 
theological grounds. So strongly does he feel this that he con- 
tinually speaks as if the planets must be inhabited unless we can 
show very good reason that they cannot be so, thus throwing the 



20 MAN'S PLACE IN THE UNIVERSE 

burden of proving a negative on his opponents, while he does 
not attempt to prove his positive contention that they are in- 
habited, except by purely hypothetical considerations as to the 
Creator's purpose in bringing them into existence. 

But starting from this point he endeavours to show how Whew- 
ell's various difficulties may be overcome, and here he always 
appeals to astronomical or physical facts, and reasons well upon 
them. But he is quite honest ; and, coming to the conclusion 
that Jupiter and Saturn, Uranus and Neptune, cannot be habi- 
table, he adduces the evidence and plainly states the result. But 
then he thinks that the satellites of Jupiter and Saturn may be 
inhabitable, and if they may be then he concludes that iheymust. 
One great oversight in his whole argument is, that he is satisfied 
with showing the possibility that life may exist now, but never 
deals with the question of whether life could have been developed 
from its earliest rudiments up to the production of the higher 
vertebrates and man ; and this, as I shall show later, is the crux 
of the whole problem. 

With regard to the other planets, after a careful examination 
of all that is known about them, he arrives at the conclusion that 
if Mercury is protected by a cloud-laden atmosphere of a peculiar 
kind it may possibly, but not probably, support high forms of 
animal life. But in the case of Venus and Mars he finds so much 
resemblance to and so many analogies with our earth, that he 
concludes that they almost certainly are so. 

In the case of the fixed stars, now that we know by spectro- 
scopic observations that they are true suns, many of which closely 
resemble our sun and give out light and heat as he does, Mr. 



MODERN IDEAS 21 

Proctor argues, that " The vast supplies of heat thus emitted 
by the stars not only suggest the conclusion that there must be 
worlds around these orbs for which these heat-supplies are in- 
tended, but point to the existence of the various forms of force 
into which heat may be transmuted. We know that the sun's 
heat poured upon our earth is stored up in vegetable and animal 
forms of life ; is present in all the phenomena of nature — in winds 
and clouds and rain, in thunder and lightning, storm and 
hail; and that even the works of man are performed by virtue 
of the solar heat-supplies. Thus the fact that the stars send forth 
heat to the worlds which circle around them, suggests at once the 
thought that on those worlds there must exist animal and veg- 
etable forms of life." We may note that in the first part of this 
passage the presence of worlds or planets is " suggested," while 
later on " the worlds which circle round them " is spoken of as if 
it were a proved fact from which the presence of vegetable and 
animal life may be inferred. A suggestion depending on a 
preceding suggestion is not a very firm basis for so vast and 
wide-reaching a conclusion. 

In the second work referred to above there is one chapter en- 
titled, " A New Theory of Life in Other Worlds," where the 
author gives his more matured views of the question, which are 
briefly stated in the preface as being " that the weight of evi- 
dence favours my theory of the [relative] paucity of worlds." 
His views are largely founded on the theory of probabilities, of 
which subject he had made a special study. Taking first our 
earth, he shows that the period during which life has existed 
upon it is very small in comparison with that during which it 



22 MAN'S PLACE IN THE UNIVERSE 

must have been slowly forming and cooling, and its atmosphere 
condensing so as to form land and water on its surface. And 
if we consider the time the earth has been occupied by man, that 
is a very minute part, perhaps not the thousandth part of the 
period during which it has existed as a planet. It follows that 
even if we consider only those planets whose physical condition 
seems to us to be such as to be able to sustain life, the chances are 
perhaps hundreds to one against theirs being at that particular 
stage when life has begun to be developed, or if it has begun has 
reached as high a development as our own earth. 

With regard to the stars the argument is still stronger, be- 
cause the epochs required for their formation are altogether 
unknown, while as to the conditions required for the formation 
of planetary systems around them we are totally ignorant. 
To this I would add that we are equally ignorant as to the prob- 
ability or even possibility of many of these suns producing 
planets which, by their position, size, atmosphere, or other physi- 
cal conditions, can possibly become life-producing worlds. And, 
as we shall see later, this point has been overlooked by all writers, 
including Mr. Proctor himself. His conclusion is, then, that 
although the worlds which possess life at all approaching that of 
our earth may be relatively few in number, yet considering the 
universe as practically infinite in extent, they may be really very 
numerous. 

It has been necessary to give this sketch of the views of those 
who have written specially on the question of the Plurality of 
Worlds, because the works referred to have been very widely read 
and have influenced educated opinion throughout the world. More- 



MODERN IDEAS 23 

over, Mr. Proctor, in his last work on the subject, speaks of the 
theory as being " identified with modern astronomy " ; and in 
fact popular works still discuss it. But all these follow the same 
general line of argument as those already referred to, and the 
curious thing is that while overlooking many of the most essential 
conditions they often introduce others which are by no means 
essential — as, for instance, that the atmosphere must have the 
same proportion of oxygen as our own. They seem to think, 
that if any of our quadrupeds or birds taken to another planet 
could not live there that no animals of equally high organisation 
could inhabit it ; entirely overlooking the very obvious fact that, 
supposing, as is almost certain, that oxygen is necessary for life, 
then, whatever proportion of oxygen within certain limits was 
present, the forms of life that arose would necessarily be organ- 
ised in adaptation to that proportion, which might be consider- 
ably less or greater than on the earth. 

The present volume will show how extremely inadequate has 
been the treatment of this question, which involves a variety of 
important considerations hitherto altogether overlooked. These 
are extremely numerous and very varied in their character, and 
the fact that they all point to one conclusion — a conclusion which 
so far as I am aware no previous writer has reached — renders it 
at least worthy of the careful consideration of all unbiassed 
thinkers. The whole subject is one as to which no direct evidence 
is obtainable, but I venture to think that the convergence of so 
many probabilities and indications towards a single definite theory 
intimately connected with the nature and destiny of man himself, 
raises this theory to a very much higher level of probability than 



24 MAN'S PLACE IN THE UNIVERSE 

the vague possibilities and theological suggestions which are the 
utmost that have been adduced by previous writers. 

In order to make every step of my argument clearly intelligible 
to all educated readers, it will be necessary to refer continually 
to the marvellous extension of our knowledge of the universe 
obtained during the last half-century, and constituting what is 
termed the New Astronomy. The next chapter will therefore be 
devoted to a popular exposition of the new methods of research, 
so that the results reached, which will have to be referred to in 
succeeding chapters, may be not only accepted, but be clearly 
understood. 



CHAPTER III 



THE NEW ASTRONOMY 



During the latter half of the nineteenth century discoveries were 
made which extended the powers of astronomical research into 
entirely new and unexpected regions, comparable to those which 
were opened up by the discovery of the telescope more than two 
centuries before. The older astronomy for more than two thou- 
sand years was purely mechanical and mathematical, being lim- 
ited to observation and measurement of the apparent motions of 
the heavenly bodies, and the attempts to deduce from these appa- 
rent motions, their real motions, and thus determine the actual 
structure of the solar system. This was first done when Kepler 
established his three celebrated laws ; and later, when Newton 
showed that these laws were necessary consequences of the one law 
of gravitation, and when succeeding observers and mathemati- 
cians proved that each fresh irregularity in the motions of the 
planets was explicable by a more thorough and minute applica- 
tion of the same laws, this branch of astronomy reached its 
highest point of efficiency and left very little more to be desired. 
Then, as the telescope became successively improved, the centre 
of interest was shifted to the surfaces of the planets and their 
satellites, which were watched and scrutinised with the greatest 
assiduity in order if possible to attain some amount of knowledge 

25 



26 MAN'S PLACE IN THE UNIVERSE 

of their physical constitution and past history. A similar 
minute scrutiny was given to the stars and nebulae, their distribu- 
tion and grouping, and the whole heavens were mapped out and 
elaborate catalogues constructed by enthusiastic astronomers in 
every part of the world. Others devoted themselves to the im- 
mensely difficult problem of determining the distances of the 
stars, and by the middle of the century a few such distances had 
been satisfactorily measured. 

Thus, up to the middle of the nineteenth century it appeared 
likely that the future of astronomy would rest almost entirely on 
the improvement of the telescope, and of the various instruments 
of measurement by means of which more accurate determinations 
of distances might be obtained. Indeed, the author of the Posi- 
tive Philosophy, Auguste Comte, felt so sure of this that he 
deprecated all further attention to the stars as pure waste of time 
that could never lead to any useful or interesting result. In his 
Philosophical Treatise on Popular Astronomy published in 
1844, he wrote very strongly on this point. He there tells us 
that, as the stars are only accessible to us by sight, they must 
always remain very imperfectly known. We can know little more 
than their mere existence. Even as regards so simple a phe- 
nomenon as their temperature this must always be inappreciable 
to a purely visual examination. Our knowledge of the stars is for 
the most part purely negative, that is, we can determine only that 
they do not belong to our system. Outside that system there 
exist, in astronomy, only obscurity and confusion, for want of 
indispensable facts ; and he concludes thus : " It is then in vain 
that for half a century it has been endeavoured to distinguish 



THE NEW ASTRONOMY 27 

two astronomies, the one solar, the other sidereal. In the eyes of 
those for whom science consists of real laws and not of incoherent 
facts, the second exists only in name, and the first alone consti- 
tutes a true astronomy ; and I am not afraid to assert that it will 
always be so." And he adds that, " all efforts directed to this 
subject for half a century have only produced an accumulation 
of incoherent empirical facts which can only interest an irrational 
curiosity." 

Seldom has a confident assertion of finality in science received 
so crushing a reply as was given to the above statements of Comte 
by the discovery in 1860 (only three years after his death) of the 
method of spectrum-analysis which, in its application to the stars, 
has revolutionised astronomy, and has enabled us to obtain that 
very kind of knowledge which he declared must be for ever beyond 
our reach. Through it we have acquired accurate information 
as to the physics and chemistry of the stars and nebulas, so that 
we now know really more of the nature, constitution, and tempera- 
ture of the enormously distant suns which we distinguish by the 
general term stars, than we do of most of the planets of our own 
system. It has also enabled us to ascertain the existence of 
numerous invisible stars, and to determine their orbits, their rate 
of motion, and even, approximately, their mass. The despised 
stellar astronomy of the early part of the century, has now taken 
rank as the most profoundly interesting department of that 
grand science, and the branch which offers the greatest promise of 
future discoveries. As the results obtained by means of this power- 
ful instrument will often be referred to, a short account of its na- 
ture and of the principles on which it depends must here be given. 



28 MAN'S PLACE IN THE UNIVERSE 

The solar spectrum is the band of coloured light seen in the 
rainbow and, partially, in the dew-drop, but more completely 
when a ray of sunlight passes through a prism — a piece of glass 
having a triangular section. The result is that instead of a spot 
of white light we have a narrow band of brilliant colours which 
succeed each other in regular order, from violet at one end, 
through blue, green, and yellow, to red at the other. We thus 
see that light is not a simple and uniform radiation from the sun, 
but is made up of a large number of separate rays, each of which 
produces in our eyes the sensation of a distinct colour. Light is 
now explained as being due to vibrations of ether, that mysterious 
substance which not only permeates all matter, but which fills 
space at least as far as the remotest of visible stars and 
nebulae. 

The exceedingly minute waves or vibrations of the ether pro- 
duce all the phenomena of heat, light, and colour, as well as those 
chemical actions to which photography owes its wonderful 
powers. By ingenious experiments the size and rate of vibra- 
tion of these waves have been measured, and it is found that they 
vary considerably, those forming the red light, which is least re- 
fracted, having a wave-length of about t^too" of an inch, 
while the violet rays at the other end of the spectrum are only 
about half that length, or -or.inro of an inch. The rate at 
which the vibrations succeed each other is from 302 millions of 
millions per second for the extreme red rays, to 737 millions of 
millions for those at the violet end of the spectrum. These 
figures are given to show the wonderful minuteness and rapidity 
of these heat and light waves, on which the whole life of the 



THE NEW ASTRONOMY 29 

world, and all our knowledge of other worlds and other suns, 
directly depend. 

But the mere colours of the spectrum are not the most im- 
portant part of it. Very early in the nineteenth century a close 
examination showed that it was everywhere crossed by black lines 
of various thicknesses, sometimes single, sometimes grouped to- 
gether. Many observers studied them and made accurate draw- 
ings or maps showing their positions and thicknesses, and by 
combining several prisms so that the beam of sunlight had to 
pass through them successively a spectrum could be produced 
several feet long, and more than 3000 of these dark lines were 
counted in it. But what they were and how they were caused 
remained a mystery, till, in the year 1860, the German physi- 
cist Kirchhoff discovered the secret and gave to chemists and 
astronomers a new and quite unexpected engine of research. 

It had already been observed that the chemical elements and 
various compounds, when heated to incandescence, produced 
spectra consisting of coloured lines or bands which were constant 
for each element, so that the elements could at once be recognised 
by their characteristic spectra ; and it had also been noticed that 
some of these bands, especially the yellow band produced by 
sodium, corresponded in position with certain black lines in the 
solar spectrum. KirchhofF's discovery consisted in showing that, 
when the light from an incandescent body passes through the 
same substance in a state of vapour or gas, so much of the light 
is absorbed that the coloured lines or bands become black. The 
mystery of more than half a century was thus solved ; and the 
thousands of black lines in the solar spectrum were shown to be 



30 MAN'S PLACE IN THE UNIVERSE 

caused by the light from the incandescent matter of the sun's sur- 
face passing through the heated gases or vapours immediately 
above it, and thereby having the bright coloured lines of their 
spectra changed, by absorption, to comparative blackness. 

Chemists and physicists immediately set to work examining the 
spectra of the elements, fixing the position of the several coloured 
lines or bands by accurate measurement, and comparing them 
with the dark lines of the solar spectrum. The results were in 
the highest degree satisfactory. In a large proportion of the 
elements the coloured bands corresponded exactly with a group 
of dark lines in the spectrum of the sun, in which, therefore, the 
same terrestrial elements were thus proved to exist. Among the 
elements first detected in this manner were hydrogen, sodium, iron, 
copper, magnesium, zinc, calcium, and many others. Nearly 
forty of the elements have nowbeen found in the sun, and it seems 
highly probable that all our elements really exist there, but as 
some are very rare and are present in very minute quantities they 
cannot be detected. Some of the dark lines in the sun were found 
not to correspond to any known element, and as this was thought 
to indicate an element peculiar to the sun it was named Helium ; 
but quite recently it has been discovered in a rare mineral. Many 
of the elements are represented by a great number of lines, others 
by very few. Thus iron has more than 2000, while lead and 
potassium have only one each. 

The value of the spectroscope both to the chemist in discover- 
ing new elements and to the astronomer in determining the con- 
stitution of the heavenly bodies, is so great, that it became of 
the highest importance to have the position of all the dark lines 



THE NEW ASTRONOMY 31 

in the solar spectrum, as well as the bright lines of all the ele- 
ments, determined with extreme accuracy so as to be able to make 
exact comparisons between different spectra. At first this was 
done by means of very large-scale drawings showing the exact 
position of every dark or bright line. But this was found to be 
both inconvenient and not sufficiently exact ; and it was therefore 
agreed to adopt the natural scale of the wave-lengths of the dif- 
ferent parts of the spectrum, which by means of what are 
termed diffraction-gratings can now be measured with great 
accuracy. Diffraction-gratings are formed of a polished surface 
of hard metal ruled with excessively fine lines, sometimes as many 
as 20,000 to an inch. When sunlight falls upon one of these 
gratings it is reflected, and by interference of the rays from the 
spaces between the fine grooves, it is spread out into a beautiful 
and well-defined spectrum which, when the lines are very close, 
is several yards in length. In these diffraction-spectra many 
dark lines are seen which can be shown in no other way, and they 
also give a spectrum which is far more uniform than that pro- 
duced by glass prisms in which minute differences in the composi- 
tion of the glass cause some rays to be refracted more and others 
less than the normal amount. 

The spectra produced by diffraction-gratings are double, that 
is, they are spread out on both sides of the central line of the 
ray which remains white, and the several coloured or dark lines 
are so clearly defined that they can be thrown on a screen at a 
considerable distance, giving a great length to the spectrum. 
The data for obtaining the wave-lengths are the distance apart 
of the lines, the distance of the screen, and the distance apart of 



32 MAN'S PLACE IN THE UNIVERSE 

the first pair of dark lines on each side of the central bright line. 
All these can be measured with extreme accuracy by means of 
telescopes with micrometers and other contrivances, and the re- 
sult is an accuracy of determination of wave-lengths which can 
probably not be equalled in any other kind of measurement. 

As the wave-lengths are so excessively minute it has been found 
convenient to fix upon a still smaller unit of measurement, and as 
the millimetre is the smallest unit of the metric system, the ten- 
millionth of a millimetre (technically termed " tenth meter ") is 
the unit adopted for the measurement of wave-lengths, which is 
equal to about the 250-millionth of an inch. Thus the wave- 
lengths of the red and blue lines characteristic of hydrogen are 
6563.07 and 4861.51 respectively. This excessively minute 
scale of wave-lengths once determined by the most refined meas- 
urement, is of very great importance. Having the wave-lengths 
of any two lines of a spectrum so determined, the space between 
them can be laid down on a diagram of any length, and all the 
lines that occur in any other spectrum between these two lines 
can be marked in their exact relative positions. Now, as the 
visible spectrum consists of about 300,000 rays of light each of 
different wave-lengths, and therefore of different refrangibilities, 
if it is laid down on such a scale as to be of a length of 3000 
inches (250 feet), each wave-length will be fiu of an inch long, 
a space easily visible by the naked eye. 

The possession of an instrument of such wonderful delicacy, 
and with powers which enable it to penetrate into the inner con- 
stitution of the remotest orbs of space, rendered it possible, within 
the next quarter of a century, to establish what is practically a 



THE NEW ASTRONOMY 33 

new science — Astrophysics — often popularly termed the New 
Astronomy. A brief outline of the main achievements of this 
science must now be given. 

The first great discovery made by Spectrum-analysis, after 
the interpretation of the sun's spectrum had been obtained, was, 
the real nature of the fixed stars. It is true they had long 
been held by astronomers to be suns, but this was only an opinion 
of the accuracy of which it did not seem possible to obtain any 
proof. The opinion was founded on two facts — their enormous 
distance from us, so great that the whole diameter of the earth's 
orbit did not lead to any apparent change of their relative posi- 
tions, and their intense brilliancy, which at such distances could 
only be due to an actual size and splendour comparable with our 
sun. The spectroscope at once proved the correctness of this 
opinion. As one after another were examined, they were found 
to exhibit spectra of the same general type as that of the sun — 
a band of colours crossed by dark lines. The very first stars 
examined by Sir William Huggins showed the existence of nine 
or ten of our elements. Very soon all the chief stars of the 
heavens were spectroscopically examined, and it was found that 
they could be classed in three or four groups. The first and 
largest group contains more than half the visible stars, and a 
still larger proportion of the most brilliant, such as Sirius, Vega, 
Regulus, and Alpha Crucis in the Southern Hemisphere. They 
are characterised by a white or bluish light, rich in the ultra- 
violet rays, and their spectra are distinguished by the breadth 
and intensity of the four dark bands due to the absorption of 
hydrogen, while the various black lines which indicate metallic 



34 MAN'S PLACE IN THE UNIVERSE 

vapours are comparatively few, though hundreds of them can be 

discovered by careful examination. 

The next group, to which Capella and Arcturus belong, is also 
very numerous, and forms the solar type of stars. Their light is 
of a yellowish colour and their spectra are crossed throughout 
by innumerable fine dark lines more or less closely corresponding 
with those in the solar spectrum. 

The third group consists of red and variable stars, which are 
characterised by fluted spectra. Such spectra show like a range 
of Doric columns seen in perspective, the red side being that 
most illuminated. 

The last group, consisting of few and comparatively small 
stars, have also fluted spectra, but the light appears to come 
from the opposite direction. 

These groups were established by Father Secchi, the Roman 
astronomer, in 1867 ; and have been adopted with some modifica- 
tions by Vogel of the Astrophysical Observatory at Potsdam. 
The exact interpretation of these different spectra is somewhat 
uncertain, but there can be little doubt that they coincide pri- 
marily with differences of temperature and with corresponding 
differences in the composition and extent of the absorptive atmos- 
pheres. Stars with fluted spectra indicate the presence of 
vapours of the metalloids or of compound substances, while the 
reversed flutings indicate the presence of carbon. These con- 
clusions have been reached by careful laboratory experiments 
which are now carried on at the same time as the spectral ex- 
amination of the stars and other heavenly bodies, so that each 
peculiarity of their spectra, however puzzling and apparently 



THE NEW ASTRONOMY 35 

unmeaning, has been usually explained by being shown to indi- 
cate certain conditions of chemical constitution or of tempera- 
ture. 

But whatever difficulty there may be in explaining details, 
there remains no doubt whatever of the fundamental fact that 
all the stars are true suns, differing no doubt in size, and their 
stage of development as indicated by the colour or intensity of 
their light or heat, but all alike possessing a photosphere or light- 
emitting surface, and absorptive atmospheres of various qualities 
and density. 

Innumerable other details, such as the often contrasted colours 
of double stars, the occasional variability of their spectra, their 
relations to the nebulae, the various stages of their development 
and other problems of equal interest, have occupied the continued 
attention of astronomers, spectroscopists, and chemists, but fur- 
ther reference to these difficult questions would be out of place 
here. The present sketch of the nature of spectrum-analysis 
applied to the stars is for the purpose of making its principle 
and method of observation intelligible to every educated reader, 
and to illustrate the marvellous precision and accuracy of the re- 
sults attained by it. So confident are astronomers of this accu- 
racy that nothing less than perfect correspondence of the various 
bright lines in the spectrum of an element in the laboratory, with 
the dark lines in the spectrum of the sun or of a star, is required 
before the presence of that element is accepted as proved. As 
Miss Clerke tersely puts it : " Spectroscopic coincidences ad- 
mit of no compromise. Either they are absolute or they are 
worthless." 



36 MAN'S PLACE IN THE UNIVERSE 

MEASUREMENT OF MOTION IN THE LINE OF SIGHT 

We must now describe another and quiet distinct application 
of the spectroscope, which is even more marvellous than that al- 
ready described. It is the method of measuring the rate of 
motion of any of the visible heavenly bodies in a direction either 
directly towards us, or directly away from us, technically de- 
scribed as " radial motion," or by the expression, " in the line of 
sight." And the extraordinary thing is that this power of 
measurement is altogether independent of distance, so that the 
rate of motion in miles per second of the remotest of the fixed 
stars, is sufficiently bright to show a distinct spectrum, can be 
measured with as much certainty and accuracy as in the case of 
a much nearer star or a planet. 

In order to understand how this is possible we have again to 
refer to the wave-theory of light ; and the analogy of other wave- 
motions will enable us better to grasp the principle on which these 
calculations depend. If on a nearly calm day we count the 
waves that pass each minute by an anchored steamboat, and then 
travel in the direction the waves come from, we shall find that a 
larger number pass us in the same time. Again, if we are stand- 
ing near a railway, and an engine comes towards us whistling, 
we shall notice that it changes its tone as it passes us ; and as it 
recedes the sound will be in a lower key, although the engine may 
be at exactly the same distance from us as when it was approach- 
ing. Yet the sound does not change to the ear of the engine- 
driver, the cause of the change being that the sound-waves reach 
us in quicker succession as the source of the waves is approaching 



THE NEW ASTRONOMY 37 

us than when it is retreating from us. Now, just as the pitch of 
a note depends upon the rapidity with which the successive air- 
vibrations reach our ear, so does the colour of a particular part 
of the spectrum depend upon the rapidity with which the ethereal 
waves which produce colour reach our eyes ; and as this rapidity 
is greater when the source of the light is approaching than when 
it is receding from us, a slight shifting of the position of the 
coloured bands, and therefore of the dark lines, will occur, as 
compared with their position in the spectrum of the sun or of 
any stationary source of light, if there is any motion sufficient 
in amount to produce a perceptible shift. 

That such a change of colour would occur was pointed out by 
Professor Doppler of Prague in 1842, and it is hence usually 
spoken of as the " Doppler principle " ; but as the changes of 
colour were so minute as to be impossible of measurement it was 
not at that time of any practical importance in astronomy. But 
when the dark lines in the spectrum were carefully mapped, and 
their positions determined with minute accuracy, it was seen that 
a means of measuring the changes produced by motion in the line 
of sight existed, since the position of any of the dark or coloured 
lines in the spectra of the heavenly bodies could be compared 
with those of the corresponding lines produced artificially in the 
laboratory. This was first done in 1868 by Sir William Hug- 
gins, who, by the use of a very powerful spectroscope constructed 
for the purpose, found that such a change did occur in the case of 
many stars, and that their rate of motion towards us or away 
from us — the radial motion — could be calculated. As the 
actual distance of some of these stars had been measured, and 



38 MAN'S PLACE IN THE UNIVERSE 

their change of position annually (their proper motion) deter- 
mined, the additional factor of the amount of motion in the 
direction of our line of sight completed the data required to fix 
their true line of motion among the other stars. The accuracy 
of this method under favourable conditions and with the best 
instruments is very great, as has been proved by those cases in 
which we have independent means of calculating the real motion. 
The motion of Venus towards or away from us can be calculated 
with great accuracy for any period, being a resultant of the com- 
bined motions of the planet and of our earth in their respective 
orbits. The radial motions of Venus were determined at the 
Lick Observatory in August and September, 1890, by spectro- 
scopic observations, and also by calculation, to be as follows: 



By Observation 


By Calculation 


Aug. 16th, 7.3 


miles 


per second 


8.1 miles 


per second 


" 22d, 8.9 


«« 


(« « 


8.2 " 


« «« 


" 30th, 7.3 


(C 


« «( 


8.3 " 


t'c u 


Sep. 3d, 8.3 


u 


u cc 


8.3 " 


<( <( 


" 4th, 8.2 


(< 


tc tt 


8.3 " 


ii « 



showing that the maximum error was only one mile per second, 
while the mean error was about a quarter of a mile. In the case 
of the stars the accuracy of the method has been tested by obser- 
vations of the same star at times when the earth's motion in its 
orbit is towards or away from the star, whose apparent radial 
velocity is, therefore, increased or diminished by a known amount. 
Observations of this kind were made by Dr. Vogel, Director of 
the Astrophysical Observatory at Potsdam, showing, in the case 
of three stars, of which ten observations were taken, a mean error 
of about two miles per second ; but as the stellar motions are more 



THE NEW ASTRONOMY 39 

rapid than those of the planets, the proportionate error is no 
greater than in the example given above. 

The great importance of this mode of determining the real 
motion of the stars is, that it gives us a knowledge of the scale 
on which such motions are progressing, and when in the course 
of time we discover whether any of their paths are rectilinear or 
curved we shall be in a position to learn something of the nature 
of the changes that are going on and of the laws on which they 
depend. 

INVISIBLE STARS AND IMPERCEPTIBLE MOTIONS 

But there is another result of this power of determining radial 
motion which is even more unexpected and marvellous, and which 
has extended our knowledge of the stars in quite a new direction. 
By its means it is possible to determine the existence of invisible 
stars and to measure the rate of otherwise imperceptible motions ; 
that is, of stars which are invisible in the most powerful modern 
telescopes, and whose motions have such a limited range that no 
telescope can detect them. 

Double or binary stars forming systems which revolve around 
their common centre of gravity were discovered by Sir William 
Herschel, and very great numbers are known ; but in most cases 
their periods of revolution are long, the shortest being about 
twelve years, while many extend to several hundred years. These 
are, of course, all visible binaries, but many are now known of 
which one star only is visible while the other is either non- 
luminous or is so close to its companion that they appear as a 
single star in the most powerful telescopes. Many of the vari- 



40 MAN'S PLACE IN THE UNIVERSE 

able stars belong to the former class, a good example of which 
is Algol in the constellation Perseus, which changes from the 
second to the fourth magnitude in about four and a half hours, 
and in about four and a half hours more regains its brilliancy till 
its next period of obscuration, which occurs regularly every two 
days and twenty-one hours. The name Algol is from the Arabic 
Al Ghoul, the familiar " ghoul " of the Arabian Nights, so 
named — " The Demon " — from its strange and weird behaviour. 
It had long been conj ectured that this obscuration was due to 
a dark companion which partially eclipsed the bright star at 
every revolution, showing that the plane of the orbit of the pair 
was almost exactly directed towards us. The application of the 
spectroscope made this conjecture a certainty. At an equal 
time before and after the obscuration, motion in the line of sight 
was shown, towards and away from us, at a rate of twenty-six 
miles per second. From these scanty data and the laws of gravi- 
tation which fix the period of revolution of planets at various 
distances from their centres of revolution, Professor Pickering of 
the Harvard Observatory was able to arrive at the following 
figures as highly probable, and which may be considered to be 
certainly not far from the truth. 



Diameter of Algol, 
Diameter of dark companion, 
Distance between their centres, 
Orbital speed of Algol, . 
Orbital speed of companion, . 
Mass of Algol, 
Mass of companion, 



1,061,000 miles. 

830,000 " 
3,230,000 " 

26.3 miles per sec. 

55.4 " " " 

4 mass of our Sun. 



When it is considered that these figures relate to a pair of stars 



THE NEW ASTRONOMY 41 

only one of which has ever been seen, that the orbital motion even 
of the visible star cannot be detected in the most powerful tele- 
scopes, and considering the enormous distance of these objects 
from us, the great results of spectroscopic observation will be 
better appreciated. 

But besides the marvel of such a discovery by such simple 
means, the facts discovered are themselves in the highest degree 
marvellous. AH that we had known of the stars through tele- 
scopic observation indicated that they were at very great dis- 
tances from each other however thickly they may appear scattered 
over the sky. This is the case even with close telescopic double 
stars owing to their enormous remoteness from us. It is now 
estimated that even stars of the first magnitude are, on a general 
average, about eighty millions of millions of miles distant ; while 
the closest double stars that can be distinctly separated by large 
telescopes are about half a second apart. These, if at the above 
distance, will be about 1500 millions of miles from each other. 
But in the case of Algol and its companion, we have two bodies 
both larger than our sun, yet with a distance of only 2% mil- 
lions of miles between their surfaces, a distance not much exceed- 
ing their combined diameters. We should not have anticipated 
that such huge bodies could revolve so closely to each other, and 
as we now know that the neighbourhood of our sun — and prob- 
ably of all suns — is full of meteoric and cometic matter, it, would 
seem probable that in the case of two suns so near together the 
quantity of such matter would be very great, and would lead 
probably by continued collisions to increase of their bulk, and 
perhaps to their final coalescence into a single giant orb. It is 



42 MAN'S PLACE IN THE UNIVERSE 

said that a Persian astronomer in the tenth century calls Algol a 
red star, while it is now white or somewhat yellowish. This 
would imply an increase of temperature caused by collisions or 
friction, and increasing proximity of the pair of stars. 

A considerable number of double stars with dark companions 
have been discovered by means of the spectroscope, although their 
motion is not directly in the line of sight, and therefore there is 
no obscuration. In order to discover such pairs the spectra of 
large numbers of stars are taken on photographic plates every 
night and for considerable periods — for a year or for several 
years. These plates are then carefully examined with a high 
magnifying power to discover any periodical displacement of the 
lines, and it is astonishing in how large a number of cases this 
has been found to exist and the period of revolution of the pair 
determined. 

But besides discovering double stars of which one is dark and 
one bright, many pairs of bright stars have been discovered by 
the same means. The method in this case is rather different. 
Each component star, being luminous, will give a separate spec- 
trum, and the best spectroscopes are so powerful that they will 
separate these spectra when the stars are at their maximum dis- 
tance although no telescope in existence, or ever likely to be made, 
can separate the component stars. The separation of the spectra 
is usually shown by the most prominent lines becoming double 
and then after a time single, indicating that the plane of revolu- 
tion is more or less obliquety towards us, so that the two stars 
if visible would appear to open out and then get nearer together 
every revolution. Then, as each star alternately approaches 



THE NEW ASTRONOMY 43 

and recedes from us the radial velocity of each can be determined 
and this gives the relative mass. In this way not only doubles, 
but triple and multiple systems have been discovered. The stars 
proved to be double by these two methods are so numerous that it 
has been estimated by one of the best observers that about one star 
in every thirteen shows inequality in its radial motion and is there- 
fore really a double star. 

THE NEBULAE 

One other great result of spectrum-analysis, and in some re- 
spects perhaps the greatest, is its demonstration of the fact that 
true nebulae exist, and that they are not all star-clusters so re- 
mote as to be irresolvable, as was once supposed. They are 
shown to have gaseous spectra, or sometimes gaseous and stellar 
spectra combined, and this, in connection with the fact that 
nebulae are frequently aggregated around nebulous stars or 
groups of stars, renders it certain that the nebulae are in no way 
separated in space from the stars, but that they constitute es- 
sential parts of one vast stellar universe. There is, indeed, good 
reason to believe that they are really the material out of which 
stars are made, and that in their forms, aggregations, and con- 
densations, we can trace the very process of evolution of stars 
and suns. 

PHOTOGRAPHIC ASTRONOMY 

But there is yet another powerful engine of research which the 
new astronomy possesses, and which, either alone or in combina- 
tion with the spectroscope, had produced and will yet produce in 



44 MAN'S PLACE IN THE UNIVERSE 

the future an amount of knowledge of the stellar universe which 
could never be attained by any other means. It has already been 
stated how the discovery of new variable and binary stars has 
been rendered possible by the preservation of the photographic 
plates on which the spectra are self -recorded, night after night, 
with every line, whether dark or coloured, in true position, so as 
to bear magnification, and by comparison with others of the series, 
enabling the most minute changes to be detected and their amount 
accurately measured. Without the preservation of such accu- 
rate records, which is in no other way possible, by far the larger 
portion of spectroscopic discoveries could never have been made. 

But there are two other uses of photography of quite a differ- 
ent nature which are equally, and perhaps in their final outcome 
may be far more, important. The first is, that by the use of the 
photographic plate the exact positions of scores, hundreds, or 
even thousands of stars can be self -mapped simultaneously with 
extreme accuracy, while any number of copies can be made of 
these star-maps. This entirely obviates the necessity for the old 
method of fixing the position of each star by repeated measure- 
ment by means of very elaborate instruments, and their registra- 
tion in laborious and expensive catalogues. So important is this 
now seen to be, that specially constructed cameras are made for 
stellar photography, and by means of the best kinds of equato- 
rial mounting are made to revolve slowly so that the image of 
each star remains stationary upon the plate for several hours. 

Arrangements have been now made among all the chief ob- 
servatories of the world to carry out a photographic survey of 
the heavens, with identical instruments so as to produce maps of 



THE NEW ASTRONOMY 45 

the whole star-system on the same scale. These will serve as 
fixed data for future astronomers, who will thus be able to deter- 
mine the movements of stars of all magnitudes with a certainty 
and accuracy hitherto unattainable. 

The other important use of photography depends upon the 
fact that with a longer exposure we increase the light-collecting 
power almost indefinitely. It will surprise many persons to learn 
that an ordinary good portrait-camera with a lens three or four 
inches in diameter, if properly mounted so that an exposure of 
several hours can be made, will show stars so minute that they 
are invisible even in the great Lick telescope. In this way the 
camera will often reveal double-stars or small groups which can 
be made visible in no other way. 

Such photographs of the stars are now constantly reproduced 
in works on Astronomy and in popular magazine articles, and 
although some of them are very striking, many persons are dis- 
appointed with them, and cannot understand their great value, 
because each star is represented by a white circle often of con- 
siderable size and with a somewhat undefined outline, not by a 
minute point of light as stars appear in a good telescope. But 
the essential matter in all such photographs is not so much the 
smallness, as the roundness of the star-images, as this proves the 
extreme precision with which the image of every star has been 
kept by the clockwork motion of the instrument on the same point 
of the plate during the whole exposure. For example, in the fine 
photograph of the Great Nebula in Andromeda, taken 29th 
December, 1888, by Dr. Isaac Roberts, with an exposure of four 
hours, there are probably over a thousand stars large and small 



46 MAN'S PLACE IN THE UNIVERSE 

to be seen, every one represented by an almost exactly circular 
wh e dot of a size dependent on the magnitude of the star. 
These round dots can be bisected by the cross hairs of a micro- 
meter with very great accuracy, and thus the distance between 
the centres of any of the pairs, as well as the direction of the line 
joining their centres, can be determined as accurately as if each 
was represented by a point only. But as a minute white speck 
would be almost invisible on the maps, and would convey no in- 
formation as to the approximate magnitude of the star, mistakes 
would be much more easily made, and it would probably be found 
necessary to surround each star with a circle to indicate its magni- 
tude, and to enable it to be easily seen. It is probable, there- 
fore, that the supposed defect is really an important advantage. 
The above-mentioned photograph is beautifully reproduced in 
Proctor's Old and New Astronomy, published after his greatly 
lamented death. 

But besides the amount of altogether new knowledge obtained 
by the methods of research here briefly explained, a great deal of 
light has been thrown on the distribution of the stars as a whole, 
and hence on the nature and extent of the stellar universe, by a 
careful study of the materials obtained by the old methods, and 
by the application of the doctrine of probabilities to the ob- 
served facts. In this way alone some very striking results have 
been reached, and these have been supported and strengthened 
by the newer methods, and also by the use of new instruments in 
the measurement of stellar distances. Some of these results bear so 
closely and directly upon the special subject of the present volume, 
that our next chapter must be devoted to a consideration of them. 



CHAPTER IV 

THE DISTRIBUTION OF THE STARS 

If we look at the heavens on a clear, moonless night in winter, 
and from a position embracing the entire horizon, the scene is 
an inexpressibly grand one. The intense sparkling brilliancy 
of Sirius, Capella, Vega, and other stars of the first magnitude; 
their striking arrangement in constellations or groups, of which 
Orion, the Great Bear, Cassiopeia, and the Pleiades, are familiar 
examples ; and the filling up between these by less and less bril- 
liant points down to the limit of vision, so as to cover the whole 
sky with a scintillating tracery of minute points of light, con- 
vey together an idea of such confused scattering and such enor- 
mous numbers, that it seems impossible to count them or to re- 
duce them to systematic order. Yet this was done for all except 
the faintest stars by Hipparchus, 134 b. c, who catalogued and 
fixed the positions of more than 1000 stars, and this is about the 
number, down to the fifth magnitude, visible in the latitude of 
Greece. A recent enumeration of all the stars visible to the naked 
eye, under the most favourable conditions and by the best eye- 
sight, has been made by the American astronomer, Pickering. 
His numbers are — for the northern hemisphere 2509, and for 
the southern hemisphere 2824, thus showing a somewhat greater 
richness in the southern celestial hemisphere. But as this differ- 

47 



48 MAN'S PLACE IN THE UNIVERSE 

ence is due entirely to a preponderance of stars between magni- 
tudes 5% and 6, that is, just on the limits of vision, while those 
down to magnitude 5% are more numerous by 85 in the northern 
hemisphere, Professor Newcomb is of opinion that there is no 
real superiority of numbers of visible stars in one hemisphere over 
the other. Again, the total number of the visible stars by the 
above enumeration is 5333. But this includes stars down to 6.2 
magnitude, while it is generally considered that magnitude 6 
marks the limit of visibility. On a re-examination of all the 
materials, the Italian astronomer Schiaparelli concludes that the 
total number of stars down to the sixth magnitude is 4303 ; and 
they seem to be about equally divided between the northern and 
southern skies. 

THE MILKY WAY 

But besides the stars themselves, a most conspicuous object 
both in the northern and southern hemispheres is that wonderful 
irregular belt of faintly diffused light termed the Milky Way or 
Galaxy. This forms a magnificent arch across the sky, best 
seen in the autumn months in our latitude. This arch while 
following the general course of a great circle round the heavens 
is extremely irregular in detail, sometimes being single, sometimes 
double, sending off occasional branches or offshoots, and also 
containing in its very midst dark rifts, spots, or patches, where 
the black background of almost starless sky can be seen through 
it. When examined through an opera-glass or small telescope 
quantities of stars are seen on the luminous background, and with 
every increase in the size and power of the telescope more and 



DISTRIBUTION OF THE STARS 49 

more stars become visible, till with the largest and best modern 
instruments the whole of the Galaxy seems densely packed with 
them, though still full of irregularities, wavy streams of stars, 
and dark rifts and patches, but always showing a faint nebulous 
background as if there remained other myriads of stars which a 
still higher optical power would reveal. 

The relations of this great belt of telescopic stars to the rest 
of the star-system have long interested astronomers, and many 
have attempted its solution. By a system of gauging, that is, 
counting all the stars that passed over the field of his telescope in 
a certain time, Sir William Herschel was the first who made a 
systematic effort to determine the shape of the stellar universe. 
From the fact that the number of stars increased rapidly as the 
Milky Way was approached from whatever direction, while in 
the Galaxy itself the numbers visible were at once more than 
doubled, he formed the idea that the shape of the entire system 
must be that of a highly compressed, very broad mass or ring 
rather less dense towards the centre where our sun was situated. 
Roughly speaking, the form was likened to a flat disc or grind- 
stone, but of irregular thickness, and split in two on one side 
where it appears to be double. The immense quantity of the 
stars which formed it was supposed to be due to the fact that we 
looked at it edgewise through an immense depth of stars ; while 
at right angles to its direction when looking towards what is 
termed the pole of the Galaxy, and also in a less degree when 
looking obliquely, we see out into space through a much thinner 
stratum of stars, which thus seem on the average to be very 
much farther apart. 



50 MAN'S PLACE IN THE UNIVERSE 

But, in the latter part of his life, Sir William Herschel realised 
that this was not the true explanation of the features presented 
by the Galaxy. The brilliant spots and patches in it, the dark 
rifts and openings, the narrow streams of light often bounded 
by equally narrow streams or rifts of darkness, render it quite 
impossible to conceive that this complex luminous ring has the 
form of a compressed disc extending in the direction in which 
we see it to a distance many times greater than its thickness. In 
one very luminous cluster Herschel thought that his telescope 
had penetrated to regions twenty times as far off as the more bril- 
liant stars forming the nearer portions of the same object. Now 
in the case of the Magellanic clouds, which are two roundish 
nebular patches of large size some distance from the Milky Way 
in the southern hemisphere and looking like detached portions 
of it, Sir John Herschel himself has shown that any such inter- 
pretation of its form is impossible ; because it requires us to sup- 
pose that in both these cases we see, not rounded masses of a 
roughly globular shape, but immensely long cones or cylinders, 
placed in such a direction that we see only the ends of them. He 
remarks that one such object so situated would be an extraor- 
dinary coincidence, but that there should be two or many such 
is altogether out of the question. But in the Milky Way there 
are hundreds or even thousands of such spots or masses of excep- 
tional brilliancy or exceptional darkness ; and, if the form of the 
Galaxy is that of a disc many times broader than thick, and 
which we see edgewise, then every one of these patches and clus- 
ters, and all the narrow winding streams of bright light or in- 
tense blackness, must be really excessively long cylinders, or tun- 



DISTRIBUTION OF THE STARS 51 

nels, or deep curving laminae, or narrow fissures. And every one 
of these, which are to be found in every part of this vast circle 
of luminosity, must be so arranged as to be exactly turned 
towards our sun. The weight of this argument, which has been 
most forcibly and clearly set forth by the late Mr. R. A. Proctor, 
in his very instructive volume Our Place among Infinities, is 
now generally admitted by astronomers, and the natural con- 
clusion is that the form of the Milky Way is that of a vast 
irregular ring, of which the section at any part is, roughly speak- 
ing, circular; while the many narrow rifts or lanes or openings 
where we seem to be able to see completely through it to the dark- 
ness of outer space beyond, render it probable that in those direc- 
tions its thickness is less instead of greater than its apparent 
width, that is, that we see the broadest side rather than the 
narrow edge of it. 

Before entering on the consideration of the relations which the 
bulk of the stars we see scattered over the entire vault of heaven 
bear to this great belt of telescopic stars, it will be advisable to 
give a somewhat full description of the Galaxy itself, both be- 
cause it is not often delineated on star-maps with sufficient accu- 
racy, or so as to show its wonderful intricacies of structure, and 
also because it constitutes the fundamental phenomenon upon 
which the argument set forth in this volume primarily rests. 
For this purpose I shall use the description of it given by Sir 
John Herschel in his Outlines of Astronomy, both because he, 
of all the astronomers of the last century, had studied it most 
thoroughly, in the northern and in the southern hemispheres, by 
eye-observation and with the aid of telescopes of great power 



62 MAN'S PLACE IN THE UNIVERSE 

and admirable quality; and also because, amid the throng of 
modern works and the exciting novelties of the last thirty years, 
his instructive volume is, comparatively speaking, very little 
known. This precise and careful description will also be of serv- 
ice to any of my readers who may wish to form a closer personal 
acquaintance with this magnificent and intensely interesting 
object, by examining its peculiarities of form and beauties of 
structure either with the naked eye, or with the aid of a good 
opera-glass, or with a small telescope of good defining power. 

A DESCRIPTION OF THE MILKY WAY 

Sir John Herschel's description is as follows : " The course of 
the Milky Way as traced through the heavens by the unaided 
eye, neglecting occasional deviations and following the line of 
its greatest brightness as well as its varying breadth and inten- 
sity will permit, conforms as nearly as the indefiniteness of its 
boundary will allow it to be fixed, to that of a great circle inclined 
at an angle of about 63° to the equinoctial, and cutting that circle 
in Right Ascension 6h. 47m. and 18h. 47m., so that its northern 
and southern poles respectively are situated in Right Ascensiow 
12h. 47m., North Polar Distance 63°, and R. A. Oh. 47m. N. P. 
D. 117°. Throughout the region where it is so remarkably sub- 
divided this great circle holds an intermediate situation between 
the two great streams ; with a nearer approximation, however, to 
the brighter and continuous stream than to the fainter and in- 
terrupted one. If we trace its course in order of right ascension, 
we find it traversing the constellation Cassiopeia, its brightest 



DISTRIBUTION OF THE STARS 63 

part passing about two degrees to the north of the star 
Delta of that constellation. Passing thence between Gamma 
and Epsilon Cassiopeia?, it sends off a branch to the south 
preceding side towards Alpha Persei, very conspicuous as far 
as that star, prolonged faintly towards Eta of the same constel- 
lation, and possibly traceable towards the Hyades and Pleiades 
as remote outliers. The main stream, however (which is here 
very faint), passes on through Auriga, over the three remark- 
able stars, Epsilon, Zeta, Eta, of that constellation called the 
Haedi, preceding Capella, between the feet of Gemini and 
the horns of the Bull (where it intersects the ecliptic nearly 
in the Solstitial Colure) and thence over the club of Orion to 
the neck of Monoceros, intersecting the equinoctial in R. A. 
6h. 54m. Up to this point from the offset in Perseus, its light 
is feeble and indefinite, but thenceforward it receives a gradual 
accession of brightness, and where it passes through the shoulder 
of Monoceros and over the head of Canis Major it presents a 
broad, moderately bright, very uniform, and, to the naked eye, 
starless stream up to the point where it enters the prow of the 
ship Argo, nearly on the southern tropic. Here it again sub- 
divides (about the star M. Puppis), sending off a narrow and 
winding branch on the preceding side as far as Gamma Argus, 
where it terminates abruptly. The main stream pursues its 
southward course to the 123d parallel of N. P. D., where it 
diffuses itself broadly and again subdivides, opening out into a 
wide fan-like expanse, nearly 20° in breadth, formed of inter- 
lacing branches, all which terminate abruptly, in a line drawn 
nearly through Lambda and Gamma Argus. 



54 MAN'S PLACE IN THE UNIVERSE 

" At this place the continuity of the Milky Way is interrupted 
by a wide gap, and where it recommences on the opposite side it 
is by a somewhat similar fan-shaped assemblage of branches 
which converge upon the bright star Eta Argus. Thence it 
crosses the hind feet of the Centaur, forming a curious and 
sharply defined semicircular concavity of small radius, and enters 
the Cross by a very bright neck or isthmus of not more than three 
or four degrees in breadth, being the narrowest portion of the 
Milky Way. After this it immediately expands into a broad 
bright mass, enclosing the stars Alpha and Beta Crucis and Beta 
Centauri, and extending almost up to Alpha of the latter con- 
stellation. In the midst of this bright mass, surrounded by it on 
all sides, and occupying about half its breadth, occurs a singular 
dark pear-shaped vacancy, so conspicuous and remarkable as to 
attract the notice of the most superficial gazer and to have ac- 
quired among the early southern navigators the uncouth but ex- 
pressive appellation of the coal-sack. In this vacancy, which is 
about 8° in length and 5° broad, only one very small star visible 
to the naked eye occurs, though it is far from devoid of telescopic 
stars, so that its striking blackness is simply due to the effect of 
contrast with the brilliant ground with which it is on all sides 
surrounded. This is the place of nearest approach of the Milky 
Way to the South Pole. Throughout all this region its bright- 
ness is very striking, and when contrasted with that of its more 
northern course already traced, conveys strongly the impression 
of greater proximity, and would almost lead to a belief that our 
situation as spectators is separated on all sides by a considerable 
interval from the dense body of stars composing the Galaxy, 



DISTRIBUTION OF THE STARS 55 

which in this view of the subject would come to be considered as a 
flat ring or some other re-entering form of immense and irregu- 
lar breadth and thickness, within which we are excentrically 
situated nearer to the southern than to the northern part of 
its circuit. 

" At Alpha Centauri the Milky Way again subdivides, send- 
ing off a great branch of nearly half its breadth, but which thins 
off rapidly, at an angle of about 20° with its general direction 
to Eta and Delta Lupi, beyond which it loses itself in a narrow 
and faint streamlet. The main stream passes on increasing in 
breadth to Gamma Normae, where it makes an abrupt elbow and 
again subdivides into one principal and continuous stream of very 
irregular breadth and brightness, and a complicated system of 
interlacing streaks and masses, which covers the tail of Scorpio, 
and terminates in a vast and faint effusion over the whole ex- 
tensive region occupied by the preceding leg of Ophiuchus, ex- 
tending northward to the parallel of 103° N. P. D., beyond 
which it cannot be traced ; a wide interval of 14°, free from all 
appearance of nebulous light, separating it from the great 
branch on the north side of the equinoctial of which it is usually 
represented as a continuation. 

" Returning to the point of separation of this great branch 
from the main stream, let us now pursue the course of the latter. 
Making an abrupt bend to the following side it passes over the 
stars Iota Aras, Theta and Iota Scorpii, and Gamma Tubi to 
Gamma Sagittarii, where it suddenly collects into a vivid oval 
mass, about 6° in length and 4° in breadth, so excessively rich in 
stars that a very moderate calculation makes their number exceed 



56 MAN'S PLACE IN THE UNIVERSE 

100,000. Northward of this mass, this stream crosses the ecliptic 
in longitude about 276°, and proceeding along the bow of Sagit- 
tarius into Antinous has its course rippled by three deep con- 
cavities, separated from each other by remarkable protuberances, 
of which the larger and brighter forms the most conspicuous 
patch in the southern portion of the Milky Way visible in our 
latitudes. 

" Crossing the equinoctial at the 19th hour of R. A., it next 
runs in an irregular, patchy, and winding stream through Aquila, 
Sagitta, and Vulpecula up to Cygnus ; at Epsilon of which con- 
stellation its continuity is interrupted, and a very confused and 
irregular region commences, marked by a broad dark vacuity, not 
unlike the southern ' coal-sack,' occupying the space between 
Epsilon, Alpha, and Gamma Cygni, which serves as a kind of 
centre for the divergence of three great streams, one of which we 
have already traced; a second, the continuation of the first 
(across the interval) from Alpha northward, between Lacerta 
and the head of Cepheus to the point in Cassiopeia whence we set 
out, and a third branching off from Gamma Cygni, very vivid 
and conspicuous, running off in a southern direction through 
Beta Cygni, and S. Aquilae almost to the equinoctial, where it 
loses itself in a region thinly sprinkled with stars, where in some 
maps the modern constellation Taurus Poniatowski is placed. 
This is the branch which, if continued across the equinoctial, 
might be supposed to unite with the great southern effusion in 
Ophiuchus already noticed. A considerable offset, or protu- 
berant appendage, is also thrown off by the northern stream 
from the head of Cepheus directly towards the pole, occupying 



DISTRIBUTION OF THE STARS 57 

the greater part of the quartile formed by Alpha, Beta, Iota, 
and Delta of that constellation." 

To complete this careful, detailed description of the Milky 
Way, it will be well to add a few passages from the same work 
as to its telescopic appearance and structure. 

" When examined with powerful telescopes, the constitution 
of this wonderful zone is found to be no less varied than its 
aspect to the naked eye is irregular. In some regions the stars 
of which it is composed are scattered with remarkable uniformity 
over immense tracts, while in others the irregularity of their dis- 
tribution is quite as striking, exhibiting a rapid succession of 
closely clustering rich patches separated by comparatively poor 
intervals, and indeed, in some instances, absolutely dark and com- 
pletely void of any star, even of the smallest telescopic magni- 
tude. In some places not more than 40 or 50 stars on an average 
occur in a 6 gauge-field of 15,' while in others a similar average 
gives a result of 400 or 500. Nor is less variety observable in 
the character of its different regions, in respect of the magni- 
tudes of the stars they exhibit, and the proportional numbers of 
the larger and smaller magnitudes associated together, than in 
respect of their aggregate numbers. In some, for instance, ex- 
tremely minute stars occur in numbers so moderate as to lead us 
irresistibly to the conclusion that in those regions we see fairly 
through the starry stratum, since it is impossible otherwise that 
the numbers of the smaller magnitudes should not go on con- 
tinually increasing ad infinitum. In such cases, moreover, the 
ground of the heavens is for the most part perfectly dark, which 
again would not be the case if innumerable multitudes of stars, 



58 MAN'S PLACE IN THE UNIVERSE 

too minute to be individually discernible, existed beyond. In 
other regions we are presented with the phenomenon of an almost 
uniform degree of brightness of the individual stars, accompanied 
with a very even distribution of them over the ground of the 
heavens, both the larger and smaller magnitudes being strikingly 
deficient. In such cases it is equally impossible not to perceive 
that we are looking through a sheet of stars nearly of a size, and 
of no great thickness compared with the distance that separates 
them from us. Were it otherwise we should be driven to suppose 
the more distant stars uniformly the larger, so as to compensate 
by their greater intrinsic brightness for their greater distance, a 
supposition contrary to all probability. . . . 

" Throughout by far the larger portion of the extent of the 
Milky Way, in both hemispheres, the general blackness of the 
ground of the heavens on which its stars are projected, and the 
absence of that innumerable multitude and excessive crowding of 
the smallest visible magnitudes, and of glare produced by the 
aggregate light of multitudes too small to affect the eye 
singly, must, we think, be considered unequivocal indications 
that its dimensions in directions where these conditions ob- 
tain, are not only not infinite, but that the space-penetrating 
power of our telescope suffices fairly to pierce through and 
beyond it." 

In the above-quoted passages the italics are those of Sir John 
Herschel himself, and we see that he drew the very same con- 
clusions from the facts he describes, and for much the same rea- 
sons, as Mr. Proctor has drawn from the observations of Sir Wil- 
liam Herschel ; and, as we shall see, the best astronomers to-day 



DISTRIBUTION OF THE STARS 69 

have arrived at a similar result, from the additional facts at their 
disposal, and in some cases from fresh lines of argument. 

THE STARS IN RELATION TO THE MILKY WAY 

Sir John Herschel was so impressed with the form, structure, 
and immensity of the Galactic Circle, as he sometimes terms it, 
that he says (in a footnote p. 575, 10th ed.), " This circle is to 
sidereal what the invariable ecliptic is to planetary astronomy — 
a plane of ultimate reference, the ground-plane of the sidereal 
system." We have now to consider what are the relations of the 
whole body of the stars to this Galactic Circle — this plane of 
ultimate reference for the whole stellar universe. 

If we look at the heavens on a starry night, the whole vault ap- 
pears to be thickly strewn with stars of various degrees of bright- 
ness, so that we could hardly say that any extensive region — the 
north, east, south, or west, or the portion vertically above us is 
very conspicuously deficient or superior in numbers. In every 
part there are to be found a fair proportion of stars of the first 
two or three magnitudes, while where these may seem deficient a 
crowd of smaller stars takes their place. 

But an accurate survey of the visible stars shows that there 
is a large amount of irregularity in their distribution, and 
that all magnitudes are really more numerous in or near the 
Milky Way, than at a distance from it, though not in so large 
a degree as to be very conspicuous to the naked eye. The area 
of the whole of the Milky Way cannot be estimated at more 
than one-seventh of the whole sphere, while some astronomers 



60 MAN'S PLACE IN THE UNIVERSE 

reckon it at only one-tenth. If stars of any particular size 
were uniformly distributed, at most one-seventh of the whole 
number should be found within its limits. But Mr. Gore finds 
that of 32 stars brighter than the second magnitude 12 lie 
upon the Milky Way, or considerably more than twice as many 
as there should be if they were uniformly distributed. And in 
the case of the 99 stars which are brighter than the third 
magnitude 33 lie upon the Milky Way, or one-third instead of 
one -seventh. Mr. Gore also counted all the stars in Heis's Atlas 
which lie upon the Milky Way, and finds there are 1186 out of a 
total of 5356, a proportion of between a fourth and a fifth in- 
stead of a seventh. 

The late Mr. Proctor in 1871 laid down on a chart two feet 
in diameter all the stars down to magnitude 9% given in Arge- 
lander's forty large charts of the stars visible in the northern 
hemisphere. They were 324,198 in number, and they distinctly 
showed by their greater density not only the whole course of the 
Milky Way, but also its more luminous portions and many of 
the curious dark rifts and vacuities, which latter are almost 
wholly avoided by these stars. 

Later on Professor Seeliger of Munich made an investigation 
of the relation of more than 135,000 stars down to the ninth 
magnitude, to the Milky Way, by dividing the whole of the 
heavens into nine regions, one and nine being circles of 20° wide 
(equal to 40° diameter) at the two poles of the Galaxy, the 
middle region, five, is a zone 20° wide including the Milky Way 
itself, and the other six intermediate zones are each 20° wide. 
The following table shows the results as given by Professor 



DISTRIBUTION OF THE STARS 61 

Newcomb, who has made some alterations in the last column of 
" Density of Stars " in order to correct differences in the esti- 
mate of magnitudes by the different authorities. 



Regions 


Area in Degrees 


Number of Stars 


Density 


I. 


1,398.7 


4,277 


2.78 


II. 


3,146.9 


10,185 


3.03 


III. 


5,126.6 


19,488 


3.54 


IV. 


4,589.8 


24,492 


5.32 


V. 


4,519.5 


33,267 


8.17 


VI. 


3,971.5 


23,580 


6.07 


VII. 


2,954.4 


11,790 


3.71 


VIII. 


1,796.6 


6,375 


3.21 


IX. 


468.2 


1,644 


3.14 



IV. B. — The inequality of the N. and S. areas is because the enumeration 
of the stars only went as far as 24° S. Decl., and therefore included only a 
part of regions VII., VIII., and IX. 

Upon this table of densities Professor Newcomb remarks as 
follows : " The star-density in the several regions increases con- 
tinuously from each pole (regions I. and IX.) to the Galaxy 
itself (region V.). If the latter were a simple ring of stars 
surrounding a spherical system of stars, the star-density would 
be about the same in regions I., II., and III., and also in VII., 
VIII., and IX., but would suddenly increase in IV. and VI. as 
the boundary of the ring was approached. Instead of such 
being the case, the numbers 2.78, 3.03, and 3.54 in the north, and 
3.14, 3.21, and 3.71 in the south, show a progressive increase 
from the galactic pole to the Galaxy itself. The conclusion 
to be drawn is a fundamental one. The universe, or at least the 
denser portion of it, is really flattened between the galactic 
poles, as supposed by Herschel and Struve." 



62 MAN'S PLACE IN THE UNIVERSE 

But looking at the series of figures in the table, and again as 
quoted by Professor Newcomb, they seem to me to show in some 



DIAGRAM OF STAR-DENSITY 



II m IT V VI TH 



is: 




From Herschel's Gauges (as given by Professor Newcomb, p. 251). 

measure what he says they do not show. I therefore drew out 
the above diagram from the figures in the table, and it certainly 
shows that the density in regions I., II., and III., and in regions 



DISTRIBUTION OF THE STARS 63 

VII., VIII., and IX., may be said to be " about the same," that 
is, they increase very slowly, and that they do " suddenly in- 
crease " in IV. and VI. as the boundary of the Galaxy is ap- 
proached. This may be explained either by a flattening towards 
the poles of the Galaxy, or by the thinning out of stars in that 
direction. 

In order to show the enormous difference of star-density in 
the Galaxy and at the galactic poles, Professor Newcomb gives 
the following table of the Herschelian gauges, on which he only 
remarks that they show an enormously increased density in the 
galactic region due to their having counted so many more stars 
there than any other observers. 

Region, . . I. II. III. IV. V. VI. VII. VIII. IX. 

Density, . . 107 154 281 560 2,019 672 261 154 111 

But an important characteristic of these figures is, that the 
Herschels alone surveyed the whole of the heavens from the 
north to the south pole, that they did this with instruments of 
the same size and quality, and that from almost life-long experi- 
ence in this particular work they were unrivalled in their power 
of counting rapidly and accurately the stars that passed over 
each field of view of their telescopes. Their results, therefore, 
must be held to have a comparative value far above those of any 
other observer or combination of observers. I have therefore 
thought it advisable to draw a diagram from their figures, and 
it will be seen how strikingly it agrees with the former diagram 
in the very slow increase of star-richness in the first three regions 



64 MAN'S PLACE IN THE UNIVERSE 

north and south, the sudden increase in regions IV. and VI. as 
we approach the Galaxy, while the only marked difference is in 
the enormously greater richness of the Galaxy itself, which is an 

DIAGRAM OF STAR-DENSITY 




From a Table in The Stars (p. 249). 



undoubtedly real phenomenon, and is brought out here by the 
unrivalled observing power of the two greatest astronomers in 
this special department that have ever lived. 

We shall find later on that Professor Newcomb himself, as the 



DISTRIBUTION OF THE STARS 



65 



DIAGRAM OF STAR-DENSITY 

Galactic 
Z»ne 




From Table in Sir J. Herschel's Outlines of Astronomy 
(10th ed., pp. 577-578). 

result of a quite different enquiry, arrives at a result in accord- 
ance with these diagrams which will then be again referred to. 



66 MAN'S PLACE IN THE UNIVERSE 

As this is a very interesting sub j ect it will be well to give another 
diagram from two tables of star-density in Sir John HerschePs 
volume already quoted. The tables are as follows : 

Zones of Galactic Average number of Stars 

North Polar Distance per field of 15° 

0° to 15° . . . » . . . 4.32 

15° to 30° . . . ♦ . . . 5.42 

30° to 45° 8.21 

45° to 60° 13.61 

60° to 75° 24.09 

75° to 90° 53.43 

0° to 15° 6.05 

15° to 30° . . . . . . 6.62 

30° to 45° 9.08 

45° to 60° 13.49 

60° to 75° 26.29 

75° to 90° . . . . . . . 59.06 

In these tables the Milky Way itself is taken as occupying 
two zones of 15° each, instead of one of 20° as in Professor 
Newcomb's tables, so that the excess in the number of stars over 
the other zones is not so large. They show also a slight pre- 
ponderance in all the zones of the southern hemisphere, but 
this is not great, and may probably be due to the clearer 
atmosphere of the Cape of Good Hope as compared with 
that of England. 

It need only be noted here that this diagram shows the same 
general features as those already given, of a continuous in- 
crease of star-density from the poles of the Galaxy, but more 
rapidly as the Galaxy itself is more nearly approached. This 
fact must, therefore, be accepted as indisputable. 



DISTRIBUTION OF THE STARS 67 

CLUSTERS AND NEBULA IN RELATION TO THE GALAXY 

An important factor in the structure of the heavens is af- 
forded by the distribution of the two classes of objects known 
as clusters and nebulas. Although we can form an almost con- 
tinuous series from double stars which revolve round their com- 
mon centre of gravity, through triple and quadruple stars, to 
groups and aggregations of indefinite extent — of which the 
Pleiades form a good example, since the six stars visible to the 
naked eye are increased to hundreds by high telescopic powers, 
while photographs with three hours' exposure show more than 
2000 stars — yet none of these correspond to the large class 
known as clusters, whether globular or irregular, which are very 
numerous, about 600 having been recorded by Sir John Herschel 
more than fifty years ago. Many of these are among the most 
beautiful and striking objects in the heavens even with a very 
small telescope or good opera-glass. Such is the luminous spot 
called Praesepe, or the Beehive, in the constellation Cancer, and 
another in the sword-handle of Perseus. 

In the southern hemisphere there is a hazy star of about the 
fourth magnitude, Omega Centauri, which with a good telescope 
is seen to be really a magnificent cluster nearly two-thirds the 
diameter of the moon, and described by Sir John Herschel as very 
gradually increasing in brightness to the centre, and composed 
of innumerable stars of the thirteenth and fifteenth magnitudes, 
forming the richest and largest object of the kind in the heavens. 
He describes it as having rings like lacework formed of the 
larger stars. By actual count, on a good photograph, there 



68 MAN'S PLACE IN THE UNIVERSE 

are more than 6000 stars, while other observers consider that 
there are at least 10,000. In the northern hemisphere one of 
the finest is that in the constellation Hercules, known as 13 
Messier. It is just visible to the naked eye or with an opera- 
glass as a hazy star of the sixth magnitude, but a good telescope 
shows it to be a globular cluster, and the great Lick telescope 
resolves even the densest central portion into distinct stars, of 
which Sir John Herschel considered there were many thousands. 
These two fine clusters are figured in many of the modern 
popular works on astronomy, and they afford an excellent idea 
of these beautiful and remarkable objects, which, when more 
thoroughly studied, will probably aid in elucidating some of the 
obscure problems connected with the constitution and develop- 
ment of the stellar universe. 

But for the purpose of the present work the most interesting 
fact connected with star-clusters is their remarkable distribution 
in the heavens. Their special abundance in and near the Milky 
Way had often been noted, but the full importance of the fact 
could not be appreciated till Mr. Proctor, and, later, Mr. Sidney 
Waters, marked down, on maps of the two hemispheres, all the 
star-clusters and nebulae in the best catalogues. The result is 
most interesting. The clusters are seen to be thickly strewn 
over the entire course of the Milky Way, and along its margins, 
while in every other part of the heavens they are thinly scattered 
at very distant intervals, with the one exception of the Magel- 
lanic clouds of the southern hemisphere, where they are again 
densely grouped; and if anything were needed to prove the 
physical connection of these clusters with the Galaxy it would 



DISTRIBUTION OF THE STARS 69 

be their occurrence in these extensive nebulous patches which 
seem like outlying portions of the Milky Way itself. With 
these two exceptions probably not one-twentieth part of the 
whole number of star-clusters are found in any part of the 
heavens remote from the Milky Way. 

Nebulae were for a long time confounded with star-clusters, 
because it was thought that with sufficient telescopic power they 
would all be resolvable into stars as in the case of the Milky 
Way itself. But when the spectroscope showed that many of 
the nebulae consisted wholly or mainly of glowing gases, while 
neither the highest powers of the best telescopes nor the still 
greater powers of the photographic plate gave any indications 
of resolvability, although a few stars were often found to be, as 
it were, entangled in them, and evidently forming part of them, 
it was seen that they constituted a distinct stellar phenom- 
enon, a view which was enforced and rendered certain by their 
quite unique mode of distribution. A few of the larger and 
irregular type, as in the case of the grand Orion nebula visible 
to the naked eye, the great spiral nebula in Andromeda, and the 
wonderful Keyhole nebula round Eta Argus, are situated in 
or near the Milky Way ; but with these and a few other excep- 
tions the overwhelming majority of the smaller irresolvable 
nebulae appear to avoid it, there being a space almost wholly free 
from nebulae along its borders, both in the northern and southern 
hemispheres ; while the great ma j ority are spread over the sky, 
far away from it in the southern hemisphere, and in the north 
clustering in a very marked degree around the galactic pole. 
The distribution of nebulae is thus seen to be the exact opposite 



70 MAN'S PLACE IN THE UNIVERSE 

to that of the star-clusters, while both are so distinctly related 
to the position of the Milky Way — the ground-plane of the 
sidereal system, as Sir John Herschel termed it — that we are 
compelled to include them all as connected portions of one grand, 
and to some extent symmetrical, universe, whose remarkable and 
opposite mode of distribution over the heavens may probably 
afford a clue to the mode of development of that universe and 
to the changes that are even now taking place within it. The 
maps referred to above are of such great importance, and are 
so essential to a clear comprehension of the nature and constitu- 
tion of the vast sidereal system which surrounds us, that I have, 
with the permission of the Royal Astronomical Society, re- 
produced them here. ( See end of volume. ) 

A careful examination of them will give a clearer idea of the 
very remarkable facts of distribution of star-clusters and nebulae 
than can be afforded by any amount of description or of 
numerical statements. 

The forms of many of the nebulae are very curious. Some 
are quite irregular, as the Orion nebula, the Keyhole nebula 
in the southern hemisphere, and many others. Some show 
a decidedly spiral form, as those in Andromeda and Canes 
Venatici ; others, again, are annular or ring-shaped, as those in 
Lyra and Cygnus, while a considerable number are termed 
planetary nebulae from their exhibiting a faint circular disc like 
that of a planet. Many have stars or groups of stars evidently 
forming parts of them, and this is especially the case with those 
of the largest size. But all these are comparatively few in 
number and more or less exceptional in type, the great majority 






DISTRIBUTION OF THE STARS 71 

being minute cloudy specks only visible with good telescopes, 
and so faint as to leave much doubt as to their exact shape and 
nature. Sir John Herschel catalogued 5000 in 1864, and more 
than 8000 were discovered up to 1890; while the application of 
the camera has so increased the numbers that it is thought there 
may really be many hundreds of thousands of them. 

The spectroscope shows the larger irregular nebulas to be gas- 
eous, as are the annular and planetary nebulas, as well as many 
very brilliant white stars ; and all these objects are most frequent 
in or near the Milky Way. Their spectra show a green line not 
produced by any terrestrial element. With the great Lick 
telescope several of the planetary nebulas have been found to be 
irregular and sometimes to be formed of compressed or looped 
rings and other curious forms. 

Many of the smaller nebulas are double or triple, but whether 
they really form revolving systems is not yet known. The 
great mass of the small nebulas that occupy large tracts of the 
heavens remote from the Galaxy are often termed irresolvable 
nebulas, because the highest powers of the largest telescopes 
show no indication of their being star-clusters, while they are 
too faint to give any definite indications of structure in the 
spectroscope. But many of them resemble comets in their 
forms, and it is thought not impossible that they may be not 
very dissimilar in constitution. 

We have now passed in review the main features presented to 
us in the heavens outside the solar system, so far as regards the 
numbers and distribution of the lucid stars (those visible to the 



72 MAN'S PLACE IN THE UNIVERSE 

naked eye) as well as those brought to view by the telescope; 
the form and chief characteristics of the Milky Way or Galaxy ; 
and lastly, the numbers and distribution of those interesting 
objects — star-clusters and nebulae in their special relations to 
the Milky Way. This examination has brought clearly before 
us the unity of the whole visible universe; that everything we 
can see, or obtain any knowledge of, with all the resources of 
modern gigantic telescopes, of the photographic plate, and of 
the even more marvellous spectroscope, forms parts of one vast 
system which may be shortly and appropriately termed the 
Stellar universe. 

In our next chapter we shall carry the investigation a step 
further, by sketching in outline what is known of the motions 
and distances of the stars, and thus obtain some important in- 
formation bearing upon our special subject of enquiry. 



CHAPTER V 

DISTANCE OF THE STARS THE SUN'S MOTION 

THROUGH SPACE 

In early ages, before any approximate idea was reached of 
the great distances of the stars from us, the simple conception 
of a crystal sphere to which these luminous points attached and 
were carried round every day on an axis near which our pole- 
star is situated, satisfied the demands for an explanation of the 
phenomena. But when Copernicus set forth the true arrange- 
ment of the heavenly bodies, earth and planets alike revolving 
round the sun at distances of many millions of miles, and when 
this scheme was enforced by the laws of Kepler and the tel- 
escopic discoveries of Galileo, a difficulty arose which astrono- 
mers were unable satisfactorily to overcome. If, said they, the 
earth revolves round the sun at a distance which cannot be less 
(according to Kepler's measurement of the distance of Mars 
at opposition) than 13% millions of miles, then how is it that 
the nearer stars are not seen to shift their apparent places when 
viewed from opposite sides of this enormous orbit? Copernicus, 
and after him Kepler and Galileo, stoutly maintained that it 
was because the stars were at such an enormous distance from 
us that the earth's orbit was a mere point in comparison. But 
this seemed wholly incredible, even to the great observer Tycho 

73 



74 MAN'S PLACE IN THE UNIVERSE 

Brahe, and hence the Copernican theory was not so generaUy 

accepted as it otherwise would have been. 

Galileo always declared that the measurement would some day 
be made, and he even suggested the method of effecting it which 
is now found to be the most trustworthy. But the sun's dis- 
tance had to be first measured with greater accuracy, and that 
was only done in the latter part of the eighteenth century by 
means of transits of Venus ; and by later observations with more 
perfect instruments it is now pretty well fixed at about 
92,780,000 miles, the limits of error being such that 92% 
millions may perhaps be quite as accurate. 

With such an enormous base-line as twice this distance, which 
is available by making observations at intervals of about six 
months when the earth is at opposite points in its orbit, it 
seemed certain that some parallax or displacement of the nearer 
stars could be found, and many astronomers with the best in- 
struments devoted themselves to the work. But the difficulties 
were enormous, and very few really satisfactory results were 
obtained till the latter half of the nineteenth century. About 
forty stars have now been measured with tolerable certainty, 
though of course with a considerable margin of possible or 
probable error, and about thirty more, which are found to have 
a parallax of one-tenth of a second or less, must be considered 
to leave a very large margin of uncertainty. 

The two nearest fixed stars are Alpha Centauri and 61 Cygni. 
The former is one of the brightest stars in the southern hemi- 
sphere, and is about 275,000 times as far from us as the sun. 
The light from this star will take ^/± years to reach us, and this 



DISTANCE OF THE STARS 75 

u light-journey," as it is termed, is generally used by astronomers 
as an easily remembered mode of recording the distances of the 
fixed stars, the distance in miles — in this case about 25 millions 
of millions — being very cumbrous. The other star, 61 Cygni, 
is only of about the fifth magnitude, yet it is the second nearest 
to us, with a light- journey of about 7% years. If we had no 
other determinations of distance than these two, the facts would 
be of the highest importance. They teach us, first, that magni- 
tude or brightness of a star is no proof of nearness to us, a fact 
of which there is much other evidence ; and in the second place, 
they furnish us with a probable minimum distance of independent 
suns from one another, which, in proportion to their sizes, 
some being known to be many times larger than our sun, 
is not more than we might expect. This remoteness may be 
partly due to those which were once nearer together having 
coalesced under the influence of gravitation. 

As this measurement of the distance of the nearer stars should 
be clearly understood by everyone who wishes to obtain some 
real comprehension of the scale of this vast universe of which 
we form a part, the method now adopted and found to be most 
effectual will be briefly explained. 

Everyone who is acquainted with the rudiments of trigonom- 
etry or mensuration, knows that an inaccessible distance can 
be accurately determined if we can measure a base line from both 
ends of which the inaccessible object can be seen, and if we have 
a good instrument with which to measure angles. The accuracy 
will mainly depend upon our base line being not excessively short 
in comparison with the distance to be measured. If it is as 



76 MAN'S PLACE IN THE UNIVERSE 

much as half or even a quarter as long the measurement may be 
as accurate as if directly performed over the ground, but if it 
is only one-hundreth or one-thousandth part as long, a very 
small error either in the length of the base or in the amount of 
the angles will produce a large error in the result. 

In measuring the distance of the moon the earth's diameter, 
or a considerable portion of it, has served as a base-line. Either 
two observers at great distances from each other, or the same 
observer after an interval of nine or ten hours, may examine the 
moon from positions six or seven thousand miles apart, and 
by accurate measurements of its angular distance from a star, 
or by the time of its passage over the meridian of the place as 
observed with a transit instrument, the angular displacement can 
be found and the distance determined with very great accuracy, 
although that distance is more than thirty times the length of 
the base. The distance of the planet Mars when nearest to us 
has been found in the same way. His distance from us even 
when at his nearest point during the most favourable opposi- 
tions is about 36 million miles, or more than four thousand times 
the earth's diameter, so that it requires the most delicate ob- 
servations many times repeated and with the finest instruments 
to obtain a tolerably approximate result. When this is done, 
by Kepler's law of the fixed proportion between the distances 
of planets from the sun and their times of revolution, the pro- 
portionate distance of all the other planets and that of the sun 
can be ascertained. This method, however, is not sufficiently 
accurate to satisfy astronomers, because upon the sun's dis- 
tance that of every other member of the solar system depends. 



DISTANCE OF THE STARS 77 

Fortunately there are two other methods by which this im- 
portant measurement has been made with much greater ap- 
proach to certainty and precision. 

The first of these methods is by means of the rare occasions 
when the planet Venus passes across the sun's disc as seen from 
the earth. When this takes place observations of the transit, 
as it is termed, are made at remote parts of the earth, the dis- 
tance between which places can of course easily be calculated 
from their latitudes and longitudes. The diagram here given 
illustrates the simplest mode of determining the sun's distance 
by this observation, and the following description from Proc- 




Diagram illustrating the transit of Venus. 



tor's Old and New Astronomy is so clear that I copy it verbally: 
" V represents Venus passing between the Earth E and the 
Sun S ; and we see how an observer at E will see Venus as at v', 
while an observer at E' will see her as at v. The measurement 
of the distance v v', as compared with the diameter of the sun's 
disc, determines the angle vVv'orEVE'; whence the distance 
E V can be calculated from the known length of the base-line 
E E\ For instance, it is known (from the known proportions 
of the Solar System as determined from the times of revolution 
by Kepler's third law) that E V bears to V v the proportion 28 



78 MAN'S PLACE IN THE UNIVERSE 

to 72, or 7 to 18 ; whence E E' bears to v v' the same proportion. 
Suppose, now, that the distance between the two stations is 
known to be 7000 miles, so that v v' is 18,000 miles ; and that 
v v' is found by accurate measurement to be ^ part of the 
sun's diameter. Then the sun's diameter, as determined by this 
observation, is 48 times 18,000 miles, or 864,000 miles; whence 
from his known apparent size, which is that of a globe 107 Va 
times farther away from us than its own diameter, his distance 
is found to be 92,736,000 miles." 

Of course, there being two observers, the proportion of the 
distance v v' to the diameter of the sun's disc cannot be measured 
directly, but each of them can measure the apparent angular 
distance of the planet from the sun's upper and lower margins 
as it passes across the disc, and thus the angular distance 
between the two lines of transit can be obtained. The distance 
v v' can also be found by accurately noting the times of the 
upper and lower passage of Venus, which, as the line of transit 
is considerably shorter in one than the other, gives by the known 
properties of the circle the exact proportion of the distance 
between them to the sun's diameter; and as this is found to be 
the most accurate method, it is the one generally adopted. For 
this purpose the stations of the observers are so chosen that the 
length of the two chords, v and v', may have a considerable 
difference, thus rendering the measurement more easy. 

The other method of determining the sun's distance is, by the 
direct measurement of the velocity of light. This was first done 
by the French physicist, Fizeau, in 1849, by the use of rapidly 
revolving mirrors, as described in most works on physics. This 



DISTANCE OF THE STARS 79 

method has now been brought to such a degree of perfection 
that the sun's distance so determined is considered to be equally 
trustworthy with that derived from the transits of Venus. The 
reason that the determination of the velocity of light leads to a 
determination of the sun's distance is, because the time taken 
by light to pass from the sun to the earth is independently 
known to be 8 min. lS 1 /^ sec. This was discovered so long ago 
as 1675 by means of the eclipses of Jupiter's satellites. These 
satellites revolve round the planet in from 1% to 16 days, and, 
owing to their moving very nearly in the plane of the ecliptic 
and the shadow of Jupiter being so large, the three which are 
nearest to the planet are eclipsed at every revolution. This 
rapid revolution of the satellites and frequency of the eclipses 
enabled their periods of recurrence to be determined with extreme 
accuracy, especially after many years of careful observation. 
It was then found that when Jupiter was at its farthest dis- 
tance from the earth the eclipses of the satellites took place a 
little more than eight minutes later than the time calculated 
from the mean period of revolution, and when the planet was 
nearest to us the eclipses occurred the same amount earlier. 
And when further observation showed that there was no differ- 
ence between calculation and observation when the planet was 
at its mean distance from us, and that the error arose and in- 
creased exactly in proportion to our varying distance from it, 
then it became clear that the only cause adequate to produce 
such an effect was, that light had not an infinite velocity but 
travelled at a certain fixed rate. This, however, though a highly 
probable explanation, was not absolutely proved till nearly two 



80 MAN'S PLACE IN THE UNIVERSE 

centuries later, by means of two very difficult measurements — 
that of the actual distance of the sun from the earth, and that 
of the actual speed of light in miles per second ; the latter cor- 
responding almost exactly with the speed deduced from the 
eclipses of Jupiter's satellites and the sun's distance as measured 
by the transits of Venus. 

But this problem of measuring the sun's distance, and 
through it the dimensions of the orbits of all the planets of our 
system, sinks into insignificance when compared with the enor- 
mous difficulties in the way of the determination of the distance 
of the stars. As a great many people, perhaps the majority of 
the readers of any popular scientific book, have little knowledge 
of mathematics and cannot realise what an angle of a minute or 
a second really means, a little explanation and illustration of 
these terms will not be out of place. An angle of one degree 
(1°) is the 360th part of a circle (viewed from its centre), the 
90th part of a right angle, the 60th part of either of the angles 
of an equilateral triangle. To see exactly how much is an angle 
of one degree we draw a short line (BC) one-tenth of an inch 



long, and from a point (A) 5% inches from it (accurately 
5.72957795 inches) we draw straight lines to B and C. Then 
the angle at A is one degree. 

Now, in all astronomical work, one degree is considered to 
be quite a large angle. Even before the invention of the tel- 
escope the old observers fixed the position of the stars and 



DISTANCE OF THE STARS 81 

planets to half or a quarter of a degree, while Mr. Proctor 
thinks that Tycho Brands positions of the stars and planets 
were correct to about one or two minutes of arc. But a minute 
of arc is obtained by dividing the line B C into sixty equal 
parts and seeing the distance between two of these with the 
naked eye from the point A. But as very long-sighted people 
can see very minute objects at 10 or 12 inches distance, we may 
double the distance A B, and then making the line B C one 
three-hundredth part of an inch long, we shall have the angle 
of one minute which Tycho Brahe was perhaps able to measure. 
How very large an amount a minute is to the modern astronomer 
is, however, well shown by the fact that the maximum difference 
between the calculated and observed positions of Uranus, which 
led Adams and Leverrier to search for and discover Neptune, 
was only 1% minutes, a space so small as to be almost invisible 
to the average eye, so that if there had been two planets, one in 
the calculated, the other in the observed place, they would have 
appeared as one to unassisted vision. 

In order now to realise what one second of arc really means, 
let us look at the circle here shown, which is as nearly as possible 
one-tenth of an inch in diameter. ( One- O -tenth of an inch.) 
If we remove this circle to a distance of 28 feet 8 inches it will 
subtend an angle of one minute, and we shall have to place it 
at a distance of nearly 1730 feet — almost one-third of a mile — 
to reduce the angle to one second. But the very nearest to us 
of the fixed stars, Alpha Centauri, has a parallax of only three- 
fourths of a second, that is, the distance of the earth from the 
sun — about 92% millions of miles — would appear no wider, 



82 MAN'S PLACE IN THE UNIVERSE 

seen from the nearest star, than does three-fourths of the above 
small circle at one-third of a mile distance. To see this circle 
at all at that distance would require a very good telescope with 
a power of at least 100, while to see any small part of it and 
to measure the proportion of that part to the whole would need 
very brilliant illumination and a large and powerful astro- 
nomical telescope. 

WHAT IS A MILLION? 

But when we have to deal with millions, and even with hun- 
dreds and thousands of millions, there is another difficulty — 
that few people can form any clear conception of what a million 
is. It has been suggested that in every large school the walls 
of one room or hall should be devoted to showing a million at one 
view. For this purpose it would be necessary to have a hundred 
large sheets of paper each about 4 feet 6 inches square, ruled in 
quarter-inch squares. In each alternate square a round black 
wafer or circle should be placed a little overlapping the square, 
thus leaving an equal amount of white space between the black 
spots. At each tenth spot a double width should be left so as to 
separate each hundred spots (10 X 10). Each sheet would then 
hold ten thousand spots, which would all be distinctly visible 
from the middle of a room 20 feet wide, each horizontal or 
vertical row containing a thousand. One hundred such sheets 
would contain a million spots, and they would occupy a space 
450 feet long in one row, or 90 feet long in five rows, so that 
they would entirely cover the walls of a room, about 30 feet 
square and 25 feet high, from floor to ceiling, allowing space 



DISTANCE OF THE STARS 83 

for doors but not for windows, the hall or gallery being lighted 
from above. Such a hall would be in the highest degree educa- 
tional in a country where millions are spoken of so glibly and 
wasted so recklessly ; while no one can really appreciate modern 
science, dealing as it does with the unimaginably great and little, 
unless he is enabled to realise by actual vision, and summing up, 
what a vast number is comprised in one of those millions, which, 
in modern astronomy and physics, he has to deal with not singly 
only, but by hundreds and thousands or even by millions. 
In every considerable town, at all events, a hall or gallery 
should have a million thus shown upon its walls. It would 
in no way interfere with the walls being covered when re- 
quired with maps, or ornamental hangings, or pictures; but 
when these were removed, the visible and countable million would 
remain as a permanent lesson to all visitors ; and I believe that it 
would have widespread beneficial effects in almost every depart- 
ment of human thought and action. On a small scale anyone 
can do this for himself by getting a hundred sheets of engineer's 
paper ruled in small squares, and making the spots very small ; 
and even this would be impressive, but not so much so as on the 
larger scale. 

In order to enable every reader of this volume at once to form 
some conception of the number of units in a million, I have made 
an estimate of the number of letters contained in it, and I find 
them to amount to about 420,000 — considerably less than half 
a million. Try and realise when reading it, that if every letter 
were a dollar, we waste as many dollars as there are letters in 
ten such volumes whenever we build a battleship. 



84 MAN'S PLACE IN THE UNIVERSE 

Having thus obtained some real conception of the immensity 
of a million, we can better realise what it must be to have every 
one of the dots above described, or every one of the letters in two 
such volumes as this, lengthened out so as to be each a mile long, 
and even then we should have reached little more than a 
hundredth part of the distance from our earth to the sun. 
When, by careful consideration of these figures, we have even 
partially realised this enormous distance, we may take the next 
step, which is, to compare this distance with that of the nearest 
fixed star. We have seen that the parallax of that star is three- 
fourths of a second, an amount which implies that the star is 
271,400 times as far from us as our sun is. If after seeing 
what a million is, and knowing that the sun is 92% times this 
distance from us in miles — a distance which itself is almost in- 
conceivable to us, we find that we have to multiply this almost 
inconceivable distance 271,400 times — more than a quarter of a 
million times — to reach the nearest of the fixed stars, we shall 
begin to realise, however imperfectly, how vast is the system of 
suns around us, and on. what a scale of immensity the material 
universe, which we see so gloriously displayed in the starry 
heavens and the mysterious galaxy, is constructed. 

This somewhat lengthy preliminary discussion is thought 
necessary in order that my readers may form some idea of the 
enormous difficulty of obtaining any measurement whatever of 
such distances. I now propose to point out what the special 
difficulties are, and how they have been overcome; and thus I 
hope to be able to satisfy them that the figures astronomers give 
us of the distances of the stars, are in no way mere guesses or 



DISTANCE OF THE STARS 85 

probabilities, but are real measurements which, within certain 
not very wide limits of error, may be trusted as giving us correct 
ideas of the magnitude of the visible universe. 

MEASUREMENT OF STELLAR DISTANCES 

The fundamental difficulty of this measurement is, of course, 
that the distances are so vast that the longest available base- 
line, the diameter of the earth's orbit, only subtends an angle 
of little more than a second from the nearest star, while for all 
the rest it is less than one second and often only a small fraction 
of it. But this difficulty, great as it is, is rendered far greater 
by the fact that there is no fixed point in the heavens from which 
to measure, since many of the stars are known to be in motion, 
and all are believed to be so in varying degrees, while the sun 
itself is now known to be moving among the stars at a rate which 
is not yet accurately determined, but in a direction which is 
fairly well known. As the various motions of the earth while 
passing round the sun, though extremely complex, are very 
accurately known, it was first attempted to determine the 
changed position of stars by observations, many times repeated 
at six months' intervals, of the moment of their passage over 
the meridian and their distance from the zenith; and then by 
allowing for all the known motions of the earth, such as pre- 
cession of the equinoxes and nutation of the earth's axis, as 
well as for refraction and for the aberration of light, to de- 
termine what residual effect was due to the difference of position 
from which the star was viewed ; and a result was thus obtained 



86 MAN'S PLACE IN THE UNIVERSE 

in several cases, though almost always a larger one than has 
been found by later observations and by better methods. These 
earlier observations, however perfect the instruments and how- 
ever skilful the observer, are liable to errors which it seems im- 
possible to avoid. The instruments themselves are subject in 
all their parts to expansion and contraction by changes of 
temperature, and when these changes are sudden one part of 
the instrument may be affected more than another, and this will 
often lead to minute errors which may seriously affect the 
amount to be measured when that is so small. Another source 
of error is due to atmospheric refraction, which is subject to 
changes both from hour to hour and at different seasons. But 
perhaps most important of all are minute changes in level of the 
foundations of the instruments even when they are carried down 
to solid rock. Both changes of temperature and changes of 
moisture of the soil produce minute alterations of level; while 
earth-tremors and slow movements of elevation or depression 
are now known to be very frequent. Owing to all these causes, 
actual measurements of differences of position at different times 
of the year, amounting to small fractions of a second, are found 
to be too uncertain for the determination of such minute angles 
with the required accuracy. 

But there is another method which avoids almost all these 
sources of error, and this is now generally preferred and adopted 
for these measurements. It is, that of measuring the distance 
between two stars situated apparently very near each other, one 
of which has large proper motion, while the other has none 
which is measurable. The proper motions of the stars were first 



DISTANCE OF THE STARS 87 

suspected by Halley in 1717, from finding that several stars, 
whose places had been given by Hipparchus, 130 b. c, were not 
in the positions where they now ought to be ; and other observa- 
tions by the old astronomers, especially those of occultations 
of stars by the moon, led to the same result. Since the time of 
Halley very accurate observations of the stars have been made, 
and in many cases it is found that they move perceptibly from 
year to year, while others move so slowly that it is only after 
forty or fifty years that the motion can be detected. The 
greatest proper motions yet determined amount to between 
7" and 8" in a year, while other stars require twenty, or even 
fifty or a hundred years to show an equal amount of displace- 
ment. At first it was thought that the brightest stars would 
have the largest proper motion, because it was supposed they 
were nearest to us, but it was soon found that many small and 
quite inconspicuous stars moved as rapidly as the most brilliant, 
while in many very bright stars no proper motion at all can be 
detected. That which moves most rapidly is a small star of 
less than the sixth magnitude. 

It is a matter of common observation that the motion of 
things at a distance cannot be perceived so well as when near, 
even though the speed may be the same. If a man is seen on the 
top of a hill several miles off, we have to observe him closely for 
some time before we can be sure whether he is walking or stand- 
ing still. But objects so enormously distant as we now know 
that the stars are, may be moving at the rate of many miles in a 
second and yet require years of observation to detect any move- 
ment at all. 



88 MAN'S PLACE IN THE UNIVERSE 

The proper motions of nearly a hundred stars have now been 
ascertained to be more than one second of arc annually, while 
a large number have less than this, and the majority have no 
perceptible motion, presumably due to their enormous distance 
from us. It is therefore not difficult in most cases to find one 
or two motionless stars sufficiently close to a star having a large 
proper motion (anything more than one-tenth of a second is so 
called) to serve as fixed points of measurement. All that is 
then required is, to measure with extreme accuracy the angular 
distance of the moving from the fixed stars at intervals of six 
months. The measurements can be made, however, on every 
fine night, each one being compared with one at nearly an in- 
terval of six months from it. In this way a hundred or more 
measurements of the same star may be made in a year, and the 
mean of the whole, allowance being made for proper motion in 
the interval, will give a much more accurate result than any 
single measurement. 

This kind of measurement can be made with extreme accu- 
racy when the two stars can be seen together in the field 
of the telescope; either by the use of a micrometer, or by 
means of an instrument called a heliometer, now often con- 
structed for the purpose. This is an astronomical telescope of 
rather large size, the ob j ect glass of which is cut in two straight 
across the centre, and the two halves made to slide upon each 
other by means of an exceedingly fine and accurate screw- 
motion, so adjusted and tested as to measure the angular dis- 
tance of two objects with extreme accuracy. This is done by 
the number of turns of the screw required to bring the two stars 



DISTANCE OF THE STARS 89 

into contact with each other, the image of each one being formed 
by one of the halves of the object glass. 

But the greatest advantage of this method of determining 
parallax is, as Sir John Herschel points out, that it gets rid 
of all the sources of error which render the older methods so 
uncertain and inaccurate. No corrections are required for pre- 
cession, nutation, or aberration, since these affect both stars 
alike, as is the case also with refraction; while alterations of 
level of the instrument have no prejudicial effect since the 
measures of angular distance taken by this method are quite 
independent of such movements. A test of the accuracy of the 
determination of parallax by this instrument is the very close 
agreement of different observers, and also their agreement 
with the new and perhaps even superior method by photog- 
raphy. 

This method was first adopted by Professor Pritchard of the 
Oxford Observatory, with a fine reflector of thirteen inches 
aperture. Its great advantage is, that all the small stars in 
the vicinity of the star whose parallax is sought, are shown in 
their exact positions upon the plate, and the distances of all of 
them from it can be very accurately measured, and by compar- 
ing plates taken at six months' intervals, each of these stars 
gives a determination of parallax, so that the mean of the whole 
will lead to a very accurate result. Should, however, the result 
from any one of these stars differ considerably from that de- 
rived from the rest, it will be due in all probability to that star 
having a proper motion of its own, and it may therefore be 
rejected. To illustrate the amount of labour bestowed by as- 



90 MAN'S PLACE IN THE UNIVERSE 

tronomers on this difficult problem, it may be mentioned that 
for the photographic measurement of the star 61 Cygni, 330 
separate plates were taken in 1886-7, and on these 30,000 
measurements of distances of the pairs of star-images were 
made. The result agreed closely with the best previous de- 
termination by Sir Robert Ball, using the micrometer, and the 
method was at once admitted by astronomers as being of the 
greatest value. 

Although, as a rule, stars having large proper motions are 
found to be comparatively near us, there is no regular propor- 
tion between these quantities, indicating that the rapidity of the 
motion of the stars varies greatly. Among fifty stars whose 
distances have been fairly well determined, the rate of actual 
motion varies from one or two up to more than a hundred miles 
per second. Among six stars with less than a tenth of a second 
of annual proper motion there is one with a parallax of nearly 
half a second, and another of one-ninth of a second, so that they 
are nearer to us than many stars which move several seconds a 
year. 

This may be due to actual slowness of motion, but is almost 
certainly caused in part by their motion being either towards 
us or away from us, and therefore only measurable by the 
spectroscope; and this had not been done when the lists of 
parallax and proper motion from which these facts are taken 
were published. It is evident that the actual direction and rate 
of motion of a star cannot be known till this radial movement, 
as it is termed — that is, towards or away from us — has been 
measured; but as this element always tends to increase the 



DISTANCE OF THE STARS 91 

visually observed rate of motion, we cannot, through its absence, 
exaggerate the actual motions of the stars. 



THE SUN S MOVEMENT THROUGH SPACE 

But there is yet another important factor which affects the 
apparent motions of all the stars — the movement of our sun, 
which, being a star itself, has a proper motion of its own. This 
motion was suspected and sought for by Sir William Herschel 
a century ago, and he actually determined the direction of its 
motion towards a point in the constellation Hercules, not very 
far removed from that fixed upon as the average of the best 
observations since made. The method of determining this 
motion is very simple, but at the same time very difficult. When 
we are travelling in a railway carriage near objects pass rapidly 
out of sight behind us, while those farther from us remain longer 
in view, and very distant objects appear almost stationary for a 
considerable time. For the same reason, if our sun is moving 
in any direction through space, the nearer stars will appear to 
travel in an opposite direction to our movement, while the more 
distant will remain quite stationary. This movement of the 
nearest stars is detected by an examination and comparison of 
their proper motions, by which it is found that in one part of 
the heavens there is a preponderance of the proper motions in 
one direction and a deficiency in the opposite direction, while in 
the directions at right angles to these the proper motions are 
not on the average greater in one direction than in the opposite. 
But the proper motions of the stars being themselves so minute, 



92 MAN'S PLACE IN THE UNIVERSE 

and also so irregular, it is only by a most elaborate mathematical 
investigation of the motions of hundreds or even of thousands 
of stars, that the direction of the solar motion can be deter- 
mined. 

Till quite recently astronomers were agreed that the motion 
was towards a point in Hercules near the outstretched arm in the 
figure of that constellation. But the latest enquiries into this 
problem, involving the comparison of the motions of several 
thousand stars in all parts of the heavens, have led to the con- 
clusion that the most probable direction of the " solar apex " 
(as the point towards which the sun is moving is termed), is in 
the adjacent constellation Lyra, and not far from the brilliant 
star Vega. This is the position which Professor Newcomb of 
Washington thinks most probable, though there is still room for 
further investigation. To determine the rate of the motion 
is very much more difficult than to fix its direction, because the 
distances of so few stars have been determined, and very few 
indeed of these lie in the directions best adapted to give accurate 
results. 

The best measurements down to 1890 led to a motion of 
about 15 miles a second. But more recently the American 
astronomer, Campbell, has determined by the spectroscope the 
motion in the line of sight of a considerable number of stars 
towards and away from the solar apex, and by comparing the 
average of these motions, he derives a motion for the sun of 
about 12% miles a second, and this is probably as near as we 
can yet reach towards the true amount. 



DISTANCE OF THE STARS 93 

SOME NUMERICAL EESULTS OF THE ABOVE MEASUREMENTS 

The measurements of distances and proper motions of a con- 
siderable number of the stars, of the motion of our sun in space 
(its proper motion), together with accurate determinations of 
the comparative brilliancy of the brightest stars as compared 
with our sun and with each other, have led to some very re- 
markable numerical results which serve as indications of the 
scale of magnitude of the stellar universe. 

The parallaxes of about fifty stars have now been repeatedly 
measured with such consistent results that Professor Newcomb 
considers them to be fairly trustworthy, and these vary from 
one-hundredth to three-quarters of a second. Three more, all 
stars of the first magnitude — Rigel, Canopus, and Alpha Cygni 
— have no measurable parallax, notwithstanding the long-con- 
tinued efforts of many astronomers, affording a striking ex- 
ample of the fact that brilliancy alone is no test of proximity. 
Six more stars have a parallax of only one-fiftieth of a second, 
and five of these are either of the first or second magnitudes. 
Of these nine stars having very small parallax or none, six 
are situated in or near to the Milky Way, another in- 
dication of exceeding remoteness, which is further shown by the 
fact that they all have a very small proper motion or none at 
all. These facts support the conclusion, which had been already 
reached by astronomers from a careful study of the distribution 
of the stars, that the larger portion of the stars of all magni- 
tudes scattered throughout the Milky Way or along its borders 
really belong to the same great system, and may be said to form 



94 MAN'S PLACE IN THE UNIVERSE 

a part of it. This is a conclusion of extreme importance be- 
cause it teaches us that the grandest of the suns, such as Rigel 
and Betelguese in the constellation Orion, Antares in the Scor- 
pion, Deneb in the Swan (Alpha Cygni), and Canopus (Alpha 
Argus), are in all probability as far removed from us as are the 
innumerable minute stars which give the nebulous or milky ap- 
pearance to the Galaxy. 

It is well to consider for a moment what these facts mean. 
Professor S. Newcomb, one of the highest authorities on these 
problems, tells us that the long series of measurements to dis- 
cover the parallax of Canopus, the brightest star in the southern 
hemisphere, would have shown a parallax of one-hundredth of a 
second, had such existed. Yet the results always seemed to 
converge to a mean of 0".000! Suppose then we assume the 
parallax of this star to be somewhat less than the hundredth of a 
second — let us say yir of a second. At the distance this 
gives, light would take almost exactly 400 years to reach us, so 
that if we suppose this very brilliant star to be situated a little 
on this side of the Galaxy, we must give to that great luminous 
circle of stars a distance of about 500 light years. We shall 
now perceive the advantage of being able to realise what a 
million really is. 

A person who had once seen a wall-space more than 100 
feet long and 20 feet high completely covered with quarter- 
inch spots a quarter of an inch apart; and then tried to 
imagine every spot to be a mile long and to be placed end to 
end in one row, would form a very different conception of a 
million miles than those who almost daily read of millions, but 



DISTANCE OF THE STARS 95 

are quite unable to visualise even one of them. Having really 
seen one million, we can partially realise the velocity of light, 
which travels over this million miles in a little less than 5% 
seconds ; and yet light takes more than 4V3 years at this in- 
conceivable speed to come to us from the very nearest of the 
stars. 

To realise this still more impressively, let us take the dis- 
tance of this nearest star, which is 26 millions of millions of 
miles. Let us look in imagination at this large and lofty hall 
covered from floor to ceiling with quarter inch spots — only one 
million. Let all these be imagined as miles. Then repeat this 
number of miles in a straight line, one after the other, as many 
times as there are spots in this hall; and even then you have 
reached only one twenty-sixth part of the distance to the nearest 
fixed star ! This million times a million miles has to be re- 
peated twenty-six times to reach the nearest fixed star; and it 
seems probable that this gives us a good indication of the dis- 
tance from each other of at least all the stars down to the sixth 
magnitude, perhaps even of a large number of the telescopic 
stars. But as we have found that the bright stars of the 
Milky Way must be at least one hundred times farther from us 
than these nearest stars we have found what may be termed a 
minimum distance for that vast star-ring. It may be immensely 
farther, but it is hardly possible that it should be anything 
less. 



96 MAN'S PLACE IN THE UNIVERSE 

THE PROBABLE SIZE OF THE STAR! 

Having thus obtained an inferior limit for the distance of 
several stars of the first magnitude, and their actual brilliancy 
or light-emission as compared with our sun, having been care- 
fully measured, we have offered us some indication of size 
though perhaps an uncertain one. By these means it has been 
found that Canopus gives out about ten thousand times as much 
light as our sun, so that if its surface is of the same brightness, 
it must be a hundred times the diameter of the sun. But as it 
is one of the white or Sirian type of stars it is probably very 
much more luminous, but even if it were twenty times brighter 
it would still have to be twenty-two and a half times the 
diameter of the sun ; and as the stars of this type are probably 
wholly gaseous and much less dense than our sun, this enormous 
size may not be far from the truth. It is believed that the Sirian 
stars generally have a greater surface brilliancy than our 
sun. 

Beta Aurigse, a star of the second magnitude but of the Sirian 
type, is one _of the double stars whose distance has been 
measured, and this has enabled Mr. Gore to find the mass of the 
binary system to be five times that of the sun, and their light 
one hundred and seventeen times greater. Even if the density 
is much less than the sun's, the intrinsic brilliancy of the surface 
will be considerably greater. Another double star, Gamma 
Leonis, has been found to be three hundred times more brilliant 
than the sun if of the same density, but it would require to be 
seven times rarer than air to have the extent of surface needed 



DISTANCE OF THE STARS 97 

to give the same amount of light if its surface emitted no more 
light than our sun from equal areas. 

It is clear, therefore, that many of the stars are much larger 
than our sun as well as more luminous ; but there are also large 
numbers of small stars whose large proper motions, as well as 
the actual measurement of some of them, prove them to be com- 
paratively near to us which yet are only about one-fiftieth part 
as bright as the sun. These must, therefore, be either com- 
paratively small, or if large must be but slightly luminous. In 
the case of some double stars it has been proved that the latter 
is the case; but it seems probable that others are very much 
smaller than the average. Up to the present time no means of 
determining the size of a star by actual measurement has been 
discovered, since their distances are so enormous that the most 
powerful telescopes show only a point of light. But now that 
we have really measured the distance of a good many stars we 
are able to determine an upper limit for their actual dimensions. 
As the nearest fixed star, Alpha Centauri, has a parallax of 
0".75, this means that if this star has a diameter as great as our 
distance from the sun (which is not much more than a hundred 
times the sun's diameter) it would be seen to have a distinct disc 
about as large as that of Jupiter's first satellite. If it were 
even one-tenth of the size supposed it would probably be seen 
as a disc in our best modern telescopes. The late Mr. Ran- 
yard remarks that if the Nebular Hypothesis is true, and our 
sun once extended as far as the orbit of Neptune, then, among 
the millions of visible suns there ought to be some now to be 
found in every stage of development. But any sun having a 



98 MAN'S PLACE IN THE UNIVERSE 

diameter at all approaching this size, and situated as far off as 
a hundred times the distance of Alpha Centauri, would be seen 
by the Lick telescope to have a disc half a second in diameter. 
Hence the fact that there are no stars with visible discs proves 
that there are no suns of the required size, and adds another 
argument, though not perhaps a strong one, against the accept- 
ance of the Nebular Hypothesis. 



CHAPTER VI 

THE UNITY AND EVOLUTION OF THE STAR SYSTEM 

The very condensed sketch now given of such of the dis- 
coveries of recent Astronomy as relate to the subject we are 
discussing will, it is hoped, give some idea both of the work 
already done and of the number of interesting problems yet 
remaining to be solved. The most eminent astronomers in 
every part of the world look forward to the solution of these 
problems not, perhaps, as of any great value in themselves, but 
as steps towards a more complete knowledge of our universe as 
a whole. Their aim is to do for the star-system what Darwin 
did for the organic world, to discover the processes of change 
that are at work in the heavens, and to learn how the mysterious 
nebula?, the various types of stars, and the clusters and systems 
of stars are related to each other. As Darwin solved the problem 
of the origin of organic species from other species, and thus 
enabled us to understand how the whole of the existing forms 
of life have been developed out of pre-existing forms, so as- 
tronomers hope to be able to solve the problem of the evolution 
of suns from some earlier stellar types, so as to be able, ulti- 
mately, to form some intelligible conception of how the whole 
stellar universe has come to be what it is. Volumes have already 
been written on this subject, and many ingenious suggestions 

ILcfC. " 



100 MAN'S PLACE IN THE UNIVERSE 

and hypotheses have been advanced. But the difficulties are very 
great ; the facts to be co-ordinated are excessively numerous, and 
they are necessarily only a fragment of an unknown whole. 
Yet certain definite conclusions have been reached; and the 
agreement of many independent observers and thinkers on the 
fundamental principles of stellar evolution, seems to assure us 
that we are progressing, if slowly yet with some established 
basis of truth, towards the solution of this, the most stupendous 
scientific problem with which the human intellect has ever at- 
tempted to grapple. 

THE UNITY OF THE STELLAR UNIVERSE 

During the latter half of the nineteenth century the opinion 
of astronomers has been tending more and more to the concep- 
tion that the whole of the visible universe of stars and nebulae 
constitutes one complete and closely related system ; and during 
the last thirty years especially the vast body of facts ac- 
cumulated by stellar research has so firmly established this view 
that it is now hardly questioned by any competent authority. 

The idea that the nebulae were far more remote from us than 
the stars, long held sway, even after it had been given up by its 
chief supporter. When Sir William Herschel, by means of his 
then unapproached telescopic power, resolved the Milky Way 
more or less completely into stars, and showed that numerous 
objects which had been classed as nebulae were really clusters of 
stars, it was natural to suppose that those which still retained 
their cloudy appearance under the highest telescopic powers 



EVOLUTION OP THE STAR SYSTEM 101 

were also clusters or systems of stars, which only needed still 
higher powers to show their true nature. This idea was supported 
by the fact that several nebulae were found to be more or less 
ring-shaped, thus corresponding on a smaller scale to the form 
of the Milky Way ; so that when Herschel discovered thousands 
of telescopic nebulas, he was accustomed to speak of them as so 
many distinct universes scattered through the immeasurable 
depths of space. 

Now, although any real conception of the immensity of the 
one stellar universe, of which the Milky Way with its associated 
stars is the fundamental feature, is, as I have shown, almost un- 
attainable, the idea of an unlimited number of other universes, 
almost infinitely remote from our own and yet distinctly visible 
in the heavens, so seized upon the imagination that it became 
almost a commonplace of popular astronomy and was not easily 
given up even by astronomers themselves. And this was in a 
large part due to the fact that Sir William Herschel's volu- 
minous writings, being almost all in the Philosophical Transac- 
tions of the Royal Society, were very little read, and that he 
only indicated his change of view by a few brief sentences which 
might easily be overlooked. The late Mr. Proctor appears to 
have been the first astronomer to make a thorough study of 
the whole of Herschel's papers, and he tells us that he read them 
all over five times before he was able thoroughly to grasp the 
writer's views at different periods. 

But the first person to point out the real teaching of the 
facts as to the distribution of the nebulas, was not an as- 
tronomer, but our greatest philosophical student of science in 



102 MAN'S PLACE IN THE UNIVERSE 

general, Herbert Spencer. In a remarkable essay on " The 
Nebular Hypothesis " in the Westminster Review of July, 
1858, he maintained that the nebulae really formed a part of our 
own Galaxy and of our own stellar universe. A single passage 
from his paper will indicate his line of argument, which, it may 
be added, had already been partially set forth by Sir John 
Herschel in his Outlines of Astronomy. 

" If there were but one nebula, it would be a curious coinci- 
dence were this one nebula so placed in the distant regions of 
space as to agree in direction with a starless spot in our own 
sidereal system. If there were but two nebulae, and both were 
so placed, the coincidence would be excessively strange. What, 
then, shall we say on finding that there are thousands of 
nebulae so placed? Shall we believe that in thousands of cases 
these far-removed galaxies happen to agree in their visible 
positions with the thin places in our own galaxy ? Such a belief 
is impossible." 

He then applies the same argument to the distribution of the 
nebulae as a whole : — " In that zone of celestial space where 
stars are excessively abundant, nebulae are rare, while in the 
two opposite celestial spaces that are farthest removed from 
this zone, nebulae are abundant. Scarcely any nebulae he near 
the galactic circle ( or plane of the Milky Way ) ; and the great 
mass of them lie round the galactic poles. Can this also be 
mere coincidence? " And he concludes, from the whole mass of 
the evidence, that " the proofs of a physical connection become 
overwhelming." 

Nothing could be more clear or more forcible; but Spencer 



EVOLUTION OF THE STAR SYSTEM 103 

not being an astronomer, and writing in a comparatively little 
read periodical, the astronomical world hardly noticed him ; and 
it was from ten to fifteen years later, when Mr. R. A. Proctor, 
by his laborious charts and his various papers read before the 
Royal and Royal Astronomical Societies from 1869 to 1875, 
compelled the attention of the scientific world, and thus did 
more perhaps than any other man to establish firmly the grand 
and far-reaching principle of the essential unity of the stellar 
universe, which is now accepted by almost every astronomical 
writer of eminence in the civilised world. 

THE EVOLUTION OF THE STELLAR UNIVERSE 

Amid the enormous mass of observations and of suggestive 
speculation upon this great and most interesting problem, it is 
difficult to select what is most important and most trustworthy. 
But the attempt must be made, because, unless my readers have 
some knowledge of the most important facts bearing upon it 
(besides those already set forth), and also learn something of 
the difficulties that meet the enquirer into causes at every step 
of his way, and of the various ideas and suggestions which have 
been put forth to account for the facts and to overcome the 
difficulties, they will not be in a position to estimate, however 
imperfectly, the grandeur, the marvel, and the mystery of the 
vast and highly complex universe in which we live and of which 
we are an important, perhaps the most important, if not the 
only permanent outcome. 



104 MAN'S PLACE IN THE UNIVERSE 

THE SUN A TYPICAL STAE 

It being now a recognised fact that the stars are suns, some 
knowledge of our own sun is an essential preliminary to an 
enquiry into their nature, and into the probable changes they 
have undergone. 

The fact that the sun's density is only one-fourth that of the 
earth, or less than one and a half times that of water, demon- 
strates that it cannot be solid, since the force of gravity at its 
surface being twenty-six and a half times that at the earth's 
surface, the materials of a solid globe would be so compressed 
that the resulting density would be at least twenty times greater 
instead of four times less than that of the earth. All the 
evidence goes to show that the body of the sun is really 
gaseous, but so compressed by its gravitative force as to be- 
have more like a liquid. A few figures as to the vast dimensions 
of the sun and the amount of light and heat emitted by it will 
enable us better to understand the phenomena it presents, and 
the interpretation of those phenomena. 

Proctor estimated that each square inch of the sun's surface 
emitted as much light as twenty-five electric arcs ; and Professor 
Langley has shown by experiment that the sun is 5300 times 
brighter, and eighty-seven times hotter than the white-hot metal 
in a Bessemer converter. The actual amount of solar heat 
received by the earth is sufficient, if wholly utilised, to keep a 
three-horsepower engine continually at work on every square 
yard of the surface of our globe. The size of the sun is such, 
that if the earth were at its centre, not only would there be 



EVOLUTION OF THE STAR SYSTEM 105 

ample space for the moon's orbit, but sufficient for another 
satellite 190,000 miles beyond the moon, all revolving inside 
the sun. The mass of matter in the sun is 745 times greater 
than that of all the planets combined, hence the powerful 
gravitative force by which they are retained in their distant 
orbits. 

What we see as the sun's surface is the photosphere or outer 
layer of gaseous or partially liquid matter kept at a definite 
level by the power of gravitation. The photosphere has a 
granular texture implying some diversity of surface or of 
luminosity; although the even contour of the sun's margin 
shows that these irregularities are not on a very large scale. 
This surface is apparently rent asunder by what are termed 
sun-spots, which were long supposed to be cavities, showing a 
dark interior; but are now thought to be due to downpours of 
cooled materials driven out from the sun, and forming the 
prominences seen during solar eclipses. They appear to be 
black, but around their margin is a shaded border or penumbra 
formed of elongated shining patches crossing and over-lapping, 
something like heaps of straw. Sometimes brilliant portions 
overhang the dark spots, and often completely bridge them 
over; and similar patches, called faculse, accompany spots, and 
in some cases almost surround them. 

Sun-spots are sometimes numerous on the sun's disc, some- 
times very few, and they are of such enormous size that when 
present they can easily be seen with the naked eye, protected by 
a piece of smoked glass ; or, better still, with an ordinary opera- 
glass similarly protected. They are found to increase in num- 



106 MAN'S PLACE IN THE UNIVERSE 

ber for several years, and then to decrease; the maxima recur- 
ring after an average period of eleven years, but with no exact- 
ness, since the interval between two maxima or minima is some- 
times only nine and sometimes as much as thirteen years ; while 
the minima do not occur midway between two maxima, but much 
nearer to the succeeding than to the preceding one. What is 
more interesting is, that variations in terrestrial magnetism fol- 
low them with great accuracy; while violent commotions in the 
sun, indicated by the sudden appearance of f aculas, sun-spots, or 
prominences on the sun's limb, are always accompanied by 
magnetic disturbances on the earth. 

WHAT SURROUNDS THE SUN 

It has been well said that what we commonly term the sun is 
really the bright spherical nucleus of a nebulous body. This 
nucleus consists of matter in the gaseous state, but so com- 
pressed as to resemble a liquid or even a viscous fluid. About 
forty of the elements have been detected in the sun by means of 
the dark lines in its spectrum, but it is almost certain that all the 
elements, in some form or other, exist there. This semi-liquid 
glowing surface is termed the photosphere, since from it are 
given out the light and heat which reach our earth. 

Immediately above this luminous surface is what is termed 
the " reversing layer " or absorbing layer, consisting of dense 
metallic vapours only a few hundred miles thick, and, though 
glowing, somewhat cooler than the surface of the photosphere. 
Its spectrum, taken at the moment when the sun is totally 



EVOLUTION OF THE STAR SYSTEM 107 

darkened, through a slit which is directed tangentially to the 
sun's limb, shows a mass of bright lines corresponding in a large 
degree to the dark lines in the ordinary solar spectrum. It is 
thus shown to be a vaporous stratum which absorbs the special 
rays emitted by each element and forming its characteristic 
coloured lines, changing them into black lines. But as coloured 
lines are not found in this layer corresponding to all the black 
lines in the solar spectrum, it is now held that special absorption 
must also occur in the chromosphere and perhaps in the corona 
itself. Sir Norman Lockyer, in his volume on Inorganic Evolu- 
tion, even goes so far as to say, that the true " reversing layer " 
of the sun — that which by its absorption produced the dark lines 
in the solar spectrum — is now shown to be not the chromosphere 
itself but a layer above it, of lower temperature. 

Above the reversing layer comes the chromosphere, a vast 
mass of rosy or scarlet emanations surrounding the sun to a 
depth of about 4000 miles. When seen during eclipses it shows 
a serrated waving outline, but subject to great changes of 
form, producing the prominences already mentioned. These 
are of two kinds, the " quiescent," which are something like 
clouds of enormous extent, and which keep their forms for a 
considerable time ; and the " eruptive," which shoot out in tower- 
ing tree-like flames or geyser-like eruptions, and while doing so 
have been shown to reach velocities of over 300 miles a second, 
and which subside again with almost equal rapidity. The 
chromosphere and its quiescent prominences appear to be truly 
gaseous, consisting of hydrogen, helium, and coronium, while 
the eruptive prominences always show the presence of metallic 



108 MAN'S PLACE IN THE UNIVERSE 

vapours, especially of calcium. Prominences increase in size 
and number in close accordance with the increase of sun-spots. 
Beyond the red chromosphere and prominences is the marvellous 
white glory of the corona, which extends to an enormous dis- 
tance round the sun. Like the prominences of the chromosphere 
it is subject to periodical changes in form and size, correspond- 
ing to the sun-spot period, but in inverse order, a minimum of 
sun-spots going with a maximum extension of the corona. At 
the total eclipse of July, 1878, when the sun's surface was 
almost wholly clear, a pair of enormous equatorial streamers 
stretched east and west of the sun to a distance of ten millions 
of miles, and lesser extensions of the corona occurred at the poles. 
At the eclipses of 1882 and 1883, on the other hand, when sun- 
spots were at a maximum, the corona was regularly stellate with 
no great extensions, but of high brilliancy. This correspond- 
ence has been noted at every eclipse, and there is therefore an 
undoubted connection between the two phenomena. 

The light of the corona is believed to be derived from three 
sources — from incandescent solid or liquid particles thrown out 
from the sun, from sunlight reflected from these particles, and 
from gaseous emissions. Its spectrum possesses a green ray, 
which is peculiar to it, and is supposed to indicate a gas named 
" coronium," in other respects the spectrum is more like 
that of reflected sunlight. The enormous extensions of the 
corona into great angular streamers seem to indicate electrical 
repulsive forces analogous to those which produce the tails 
of comets. 

Connected with the sun's corona is that strange phenomenon, 



EVOLUTION OF THE STAR SYSTEM 109 

the zodiacal light. This is a delicate nebulosity, which is often 
seen after sunset in spring and before sunrise in autumn, taper- 
ing upwards from the sun's direction along the plane of the 
ecliptic. Under very favourable conditions it has been traced 
in the eastern sky in spring to 180° from the sun's position, 
indicating that it extends beyond the earth's orbit. Long- 
continued observations from the summit of the Pic du Midi 
show that this is really the case, and that it lies almost exactly 
in the plane of the sun's equator. It is therefore held to be 
produced by the minute particles thrown off the sun, through 
those coronal wings and streamers which are visible only during 
solar eclipses. 

The careful study of the solar phenomena has very clearly 
established the fact that none of the sun's envelopes, from the 
reversing layer to the corona itself, is in any sense an atmos- 
phere. The combination of enormous gravitative force with 
an amount of heat which turns all the elements into the liquid 
or gaseous state, leads to consequences which it is difficult for 
us to follow or comprehend. There is evidently constant in- 
ternal movement or circulation in the interior of the sun, re- 
sulting in the faculae, the sun-spots, the intensely luminous 
photosphere, and the chromosphere with its vast flaming 
coruscations and eruptive protuberances. But it seems impos- 
sible that this incessant and violent movement can be kept up 
without some great and periodical or continuous inrush of fresh 
materials to renew the heat, keep up the internal circulation, 
and supply the waste. Perhaps the movement of the sun 
through space may bring him into contact with sufficiently large 



110 MAN'S PLACE IN THE UNIVERSE 

masses of matter continually to excite that internal movement 
without which the exterior surface would rapidly become cool 
and all planetary life cease. The various solar envelopes are 
the result of this internal agitation, uprushes, and explosions, 
while the vast white corona is probably of little more density 
than comets' tails, probably even of less density, since comets 
not unfrequently rush through its midst without suffering any 
loss of velocity. The fact that none of the solar envelopes are 
visible to us until the light of the photosphere is completely shut 
off, and that they all vanish the very instant the first gleam of 
direct sunlight reaches us, is another proof of their extreme 
tenuity, as is also the sharply defined edge of the sun's disc. 
The envelopes therefore consist partly of liquid or vaporous 
matter, in a very finely divided state, driven off by explosions 
or by electrical forces, and this matter, rapidly cooling, becomes 
solidified into minutest particles, or even physical molecules. 
Much of this matter continually falls back on the sun's surface, 
but a certain quantity of the very finest dust is continually 
driven away by electrical repulsion, so as to form the corona 
and the zodiacal light. The vast coronal streamers and the 
still more extensive ring of the zodiacal light are therefore in 
all probability due to the same causes, and have a similar 
physical constitution as the tails of comets. 

As the whole of our sunlight must pass through both the 
reversing layer and the red chromosphere, its colour must be 
somewhat modified by them. Hence it is believed that, if they 
were absent, not only would the light and heat of the sun be 
considerably greater, but that its colour would be a purer 



EVOLUTION OF THE STAR SYSTEM 111 

white, tending towards bluish rather than towards the yellowish 
tinge it actually possesses. 

THE NEBULAE AND METEOEITIC HYPOTHESES 

As the constitution of the sun, and its agency in producing 
magnetism and electricity in the matter and orbs around it, 
afford us our best guide to the constitution of the stars and 
nebulae, and to their possible action on each other, and even upon 
our earth, so the mode of evolution of the sun and solar system, 
from some pre-existing condition, is likely to help us towards 
gaining some knowledge of the constitution of the stellar 
universe and the processes of change going on there. 

At the very commencement of the nineteenth century the 
great mathematician Laplace published his Nebular Theory of 
the Origin of the Solar System; and although he put it forth 
merely as a suggestion, and did not support it with any 
numerical or physical data, or by any mathematical processes, his 
great reputation, and its apparent probability and simplicity, 
caused it to be almost universally accepted, and to be extended 
so as to apply to the evolution of the stellar universe. This 
theory, very briefly stated, is, that the whole of the matter of 
the solar system once formed a globular or spheroidal mass of 
intensely heated gases, extending beyond the orbit of the outer- 
most planet, and having a slow motion of revolution about an 
axis. As it cooled and contracted its rate of revolution in- 
creased, and this became so great that at successive epochs it 
threw off rings, which, owing to slight irregularities, broke up, 



112 MAN'S PLACE IN THE UNIVERSE 

and gravitating together formed the planets. The contraction 
continuing, the sun, as we now see it, was the result. 

For about half a century this nebular hypothesis was 
generally accepted, but during the last thirty years so many 
objections and difficulties have been suggested, that it has been 
felt impossible to retain it even as a working hypothesis. At 
the same time another hypothesis has been put forth which seems 
more in accordance with the facts of nature as we find them in 
our own solar system, and which is not open to any of the 
objections against the nebular theory, even if it introduces a 
few new ones. 

A fundamental objection to Laplace's theory is, that in a 
gas of such extreme tenuity as the solar nebula must have been, 
even when it extended only to Saturn or Uranus, it could not 
possibly have had any cohesion, and therefore could not have 
given off whole rings at distant intervals, but only small frag- 
ments continuously as condensation went on, and these, rapidly 
cooling, would form solid particles, a kind of meteoric dust,, 
which might aggregate into numerous small planets, or might 
persist for indefinite periods, like the rings of Saturn or the 
great ring of the Asteroids. 

Another equally vital objection is, that, as the nebula when 
extending beyond the orbit of Neptune could have had a mean 
density of only about the two-hundred-millionth of our air at 
sea level, it must have been many hundred times less dense than 
this at and near its outer surface, and would there be exposed 
to the cold of stellar space — a cold that would solidify hydro- 
gen. It is thus evident that the gases of all the metallic and 



EVOLUTION OF THE STAR SYSTEM 113 

other solid elements could not possibly exist as such, but would 
rapidly, perhaps almost instantaneously, become first liquid and 
then solid, forming meteoric dust even before contraction had 
gone far enough to produce such increased rotation as would 
throw off any portion of the gaseous matter. 

Here we have the foundations of the meteoritic hypothesis 
which is now steadily making its way. It is supported by the 
fact that we everywhere find proofs of such solid matter in the 
planetary spaces around us. It falls continually upon the 
earth. It can be collected on the Arctic and Alpine snows. It 
occurs everywhere in the deepest abysses of the ocean where 
there are not sufficient organic deposits to mask it. It con- 
stitutes, as has now been demonstrated, the rings of Saturn. 
Thousands of vast rings of solid particles circulate around the 
sun, and when our earth crosses any of these rings, and their 
particles enter our atmosphere with planetary velocity, the 
friction ignites them and we see falling stars. Comets' tails, 
the sun's corona, and the zodiacal light are three strange 
phenomena, which, though wholly insoluble on any theory of 
gaseous formation, receive their intelligible explanation by 
means of excessively minute solid particles — microscopic cosmic 
dust — driven outward by the tremendous electrical repulsions 
that emanate from the sun. 

Having these and other proofs that solid matter, ranging in 
size, perhaps, from the majestic orbs of Jupiter and Saturn 
down to the inconceivably minute particles driven millions of 
miles into space to form a comet's tail, does actually exist every- 
where around us, and by collisions between the particles or with 



114 MAN'S PLACE IN THE UNIVERSE 

planetary atmospheres can produce heat and light and gaseous 
emanations, we find a basis of fact and observation for the 
meteoritic hypothesis which Laplace's nebular, and essentially 
gaseous, theory does not possess. 

During the latter half of the nineteenth century several 
writers suggested this idea of the possible formation of the 
Solar System, but so far as I am aware, the late It. A. Proctor 
was the first to discuss it in any detail, and to show that it ex- 
plained many of the peculiarities in the size and arrangement 
of the planets and their satellites which the nebular hypothesis 
did not explain. This he does at some length in the chapter 
on meteors and comets in his Other Worlds than Ours, published 
in 1870. He assumed, instead of the fire-mist of Laplace, that 
the space now occupied by the solar system, and for an un- 
known distance around it, was occupied by vast quantities of 
solid particles of all the kinds of matter which we now find in 
the earth, sun, and stars. This matter was dispersed somewhat 
irregularly, as we see that all the matter of the universe is now 
distributed; and he further assumed that it was all in motion, 
as we now know that all the stars and other cosmical masses are, 
and must be, in motion towards or around some centre. 

Under these conditions, wherever the matter was most ag- 
gregated, there would be a centre of attraction through gravita- 
tion, which would necessarily lead to further aggregation, and 
the continual impacts of such aggregating matter would pro- 
duce heat. In course of time, if the supply of cosmic matter 
was ample (as the result shows that it must have been, whatever 
theory we adopt), our sun, thus formed, would approximate 



EVOLUTION OF THE STAR SYSTEM 115 

to its present mass and acquire sufficient heat by collision and 
gravitation to convert its whole body into the liquid or gaseous 
condition. While this was going on, subordinate centres of 
aggregation might form, which would capture a certain propor- 
tion of the matter flowing in under the attraction of the central 
mass, while, owing to the nearly uniform direction and velocity 
with which the whole system was revolving, each subordinate 
centre would revolve around the central mass, in somewhat 
different planes, but all in the same direction. 

Mr. Proctor shows the probability that the largest outside 
aggregation would be at a great distance from the central mass, 
and this having once been formed, any centres further away from 
the sun would be both smaller and very remote, while those in- 
side the first would, as a rule, become smaller as they were nearer 
the centre. The heated condition of the earth's interior would 
thus be due, not to the primitive heat of matter in a gaseous 
state out of which it was formed — a condition physically im- 
possible — but would be acquired in the process of aggregation 
by the collisions of meteoric masses falling on it, and by its 
own gravitative force producing continuous condensation and 
heat. 

On this view Jupiter would probably be formed first, and 
after him at very great distances, Saturn, Uranus, and 
Neptune; while the inner aggregations would be smaller, 
as the much greater attractive power of the sun would give 
them comparatively little opportunity of capturing the meteoric 
matter that was continuously flowing towards him. 



116 MAN'S PLACE IN THE UNIVERSE 

THE METEORIC NATURE OF THE NEBULA 

Having thus reached the conclusion that wherever apparently 
nebulous matter exists within the limits of the solar system it 
is not gaseous but consists of solid particles, or, if heated gases 
are associated with the solid matter they can be accounted for 
by the heat due to collisions either with other solid particles or 
with accumulations of gases at a low temperature, as when 
meteorites enter our atmosphere, it was an easy step to consider 
whether the cosmic nebulae and stars may not have had a similar 
origin. 

From this point of view the nebulae are supposed to be vast 
aggregations of meteorites or cosmic dust, or of the more 
persistent gases, revolving with circular or spiral motions, or 
in irregular streams, and so sparsely scattered that the separate 
particles of dust may be miles — perhaps hundreds of miles — 
apart ; yet even those nebulae, only visible by the telescope, 
may contain as much matter as the whole solar system. 
From this simple origin, by steps which can be observed 
in the skies, almost all the forms of suns and systems 
can be traced by means of the known laws of motion, of heat- 
production, and of chemical action. The chief English 
advocate of this view at the present time is Sir Norman Lockyer, 
who, in numerous papers, and in his works on The Meteoritic 
Hypothesis and Inorganic Evolution, has developed it in detail, 
as the result of many years' continuous research, aided by the 
contributory work of Continental and American astronomers. 
These views are gradually spreading among astronomers and 



EVOLUTION OF THE STAR SYSTEM 117 

mathematicians, as will be seen by the very brief outline which 
will now be given of the explanations they afford of the main 
groups of phenomena presented by the stellar universe. 

DR. ROBERTS ON SPIRAL NEBULA 

Dr. Isaac Roberts, who possesses one of the finest telescopes 
constructed for photographing stars and nebulae, has given his 
views on stellar evolution, in Knowledge of February, 1897, 
illustrated by four beautiful photographs of spiral nebulae. 
These curious forms were at first thought to be rare, but are 
now found to be really very numerous when details are brought 
out by the camera. Many of the very large and apparently 
quite irregular nebulas, like the Magellanic Clouds, are found 
to have faint indications of spiral structure. As more than ten 
thousand nebulae are now known, and new ones are continually 
being discovered, it will be a long time before these can all be 
carefully studied and photographed, but present indications 
seem to show that a considerable proportion of them will exhibit 
spiral forms. 

Dr. Roberts tells us that all the spiral nebulae he has photo- 
graphed are characterised by having a nucleus surrounded by 
dense nebulosity, most of them being also studded with stars. 
These stars are always arranged more or less symmetrically, 
following the curves of the spiral, while outside the visible 
nebula are other stars arranged in curves strongly suggesting a 
former greater extension of the nebulous matter. This is so 
marked a feature that it at once leads to a possible explanation of 



118 MAN'S PLACE IN THE UNIVERSE 

the numerous slightly curved lines of stars found in every 

part of the heavens, as being the result of their origin from 

spiral nebulae whose material substance has been absorbed by 

them. 

Dr. Roberts proposes several problems in relation to these 
bodies : Of what materials are spiral nebulae composed? Whence 
comes the vortical motion which has produced their forms? 
The material he finds in those faint clouds of nebulous matter, 
often of vast extent, that exist in many parts of the sky, and 
these are so numerous that Sir William Herschel alone recorded 
the positions of fifty-two such regions, many of which have 
been confirmed by recent photographs. Dr. Roberts considers 
these to be either gaseous or with discrete solid particles inter- 
mixed. He also enumerates smaller nebulous masses under- 
going condensation and segregation into more regular forms; 
spiral nebulae in various stages of condensation and of aggrega- 
tion; elliptic nebulae; and globular nebulae. In the last three 
classes there is clear evidence, on every photograph that has 
been taken, that condensation into stars or starlike forms is now 
going on. 

He adopts Sir Norman Lockyer's view that collisions of 
meteorites within each swarm or cloud would produce luminous 
nebulosity ; so also would collisions between separate swarms of 
meteorites produce the conditions required to account for the 
vortical motions and the peculiar distribution of the nebulosity 
in the spiral nebulae. Almost any collision between unequal 
masses of diffused matter would, in the absence of any massive 
central body round which they would be forced to revolve, lead 



EVOLUTION OF THE STAR SYSTEM 119 

to spiral motions. It is to be noted that, although the stars 
formed in the spiral convolutions of the nebulae follow those 
curves, and retain them after the nebulous matter has been all 
absorbed by them, yet, whenever such a nebula is seen by us 
edgewise, the convolutions with their enclosed stars will appear 
as straight lines; and thus not only numbers of star groups 
arranged in curves, but also those which form almost perfect 
straight lines, may possibly be traced back to an origin from 
spiral nebula. 

Motion being a necessary result of gravitation, we know that 
every star, planet, comet, or nebula, must be in motion through 
space, and these motions — except in systems physically con- 
nected or which have had a common origin — are, apparently, 
in all directions. How these motions originated and are now 
regulated we do not know ; but there they are, and they furnish 
the motive power of the collisions, which, when affecting large 
bodies or masses of diffused matter, lead to the formation of the 
various kinds of permanent stars ; while when smaller masses of 
matter are concerned those temporary stars are formed which 
have interested astronomers in all ages. It must be noted that 
although the motions of the single stars appear to be in straight 
lines, yet the spaces through which they have been observed to 
move are so small that they may really be moving in curved 
orbits around some central body, or the centre of gravity of 
some aggregation of stars bright and dark, which may itself be 
comparatively at rest. There may be thousands of such centres 
around us, and this may sufficiently explain the apparent 
motions of stars in all directions. 



120 MAN'S PLACE IN THE UNIVERSE 

A SUGGESTION AS TO ,XHE FORMATION OF SPIRAL NEBULA 

In a remarkable paper in the Astrophysical Journal (July, 
1901), Mr. T. C. Chamberlin suggests an origin for the spiral 
nebulae, as well as of swarms of meteorites and comets, which 
seems likely to be a true, although perhaps not the only one. 

There is a well-known principle which shows that when two 
bodies in space, of stellar size, pass within a certain distance of 
each other, the smaller one will be liable to be torn into frag- 
ments by the differential attraction of the larger and denser 
body. This was originally proved in the case of gaseous and 
liquid bodies, and the distance within which the smaller one will 
be disrupted (termed the Roche limit) is calculated on the sup- 
position that the disrupted body is a liquid mass. Mr. Cham- 
berlin shows, however, that a solid body will also be disrupted at 
a lesser distance dependent on its size and cohesive strength ; but, 
as the size of the two bodies increases, the distance at which 
disruption will occur increases also till with very large bodies, 
such as suns, it becomes almost as large as in the case of liquids 
or gases. 

The disruption occurs from the well-known law of differential 
gravitation on the two sides of a body leading to tidal deforma- 
tion in a liquid, and to unequal strain in a solid. When the 
changes of gravitative force take place slowly, and are also 
small in amount, the tides in liquids or strains in solids are very 
small, as in the case of our earth when acted on by the sun and 
moon, the result is a small tide in the ocean and atmosphere, 
and no doubt also in the molten interior, to which the com- 



EVOLUTION OF THE STAR SYSTEM 121 

paratively thin crust may partially adjust itself. But if we 
suppose two dark or luminous suns whose proper motions are 
in such a direction as to bring them near each other, then, as 
they approach, each will be deflected towards the other, and will 
pass round their common centre of gravity with immense 
velocity, perhaps hundreds of miles in a second. At a con- 
siderable distance they will begin to produce tidal elongation 
towards and away from each other, but when the disruptive 
limit is nearly reached, the gravitative forces will be increasing 
so rapidly that even a liquid mass could not adjust its 
shape with sufficient quickness and the tremendous internal 
strains would produce the effects of an explosion, tearing 
the whole mass (of the smaller of the two) into fragments 
and dust. 

But it is also shown that, during the entire process, the two 
elongated portions of the originally spherical mass would be so 
acted upon by gravity as to produce increasing rotation, which 
as the crisis approached would extend the elongation, and aid 
in the explosive result. This rapid rotation of the elongated 
.mass, would, when the disruption occurred, necessarily give to 
the fragments a whirling or spiral motion, and thus initiate a 
spiral nebula of a size and character dependent on the size and 
constitution of the two masses, and on the amount of the 
explosive forces set up by their approach. 

There is one very suggestive phenomenon which seems to 
prove that this is one of the modes of formation of spiral 
nebulae. When the explosive disruption occurs the two protu- 
berances or elongations of the body will fly apart, and having 



122 MAN'S PLACE IN THE UNIVERSE 

also a rapid rotatory movement, the resulting spiral will neces- 
sarily be a double one. Now, it is the fact that almost all the 
well-developed spiral nebulae have two such arms opposite to 
each other, as beautifully shown in M. 100 Comas, M. 51 Canum, 
and others photographed by Dr. I. Roberts. It does not seem 
likely that any other origin of these nebulae should give rise to a 
double rather than to a single spiral. 

THE EVOLUTION OF DOUBLE STARS 

The advance in knowledge of double and multiple stars has 
been wonderfully rapid, numerous observers having devoted 
themselves to this special branch. Many thousands were dis- 
covered during the first half of the nineteenth century, and as 
telescopic power increased new ones continued to flow in by 
hundreds and thousands, and there has been recently published 
by the Yerkes Observatory a catalogue of 1290 such stars, dis- 
covered between 1871 and 1899 by one observer, Mr. S. W. 
Barnham. All these have been found by the use of the tele- 
scope, but during the last quarter of a century the spectroscope 
has opened up a new world of double stars of enormous extent 
and the highest interest. 

The telescopic binaries which have been observed for a suf- 
ficent time to determine their orbits, range from periods of about 
eleven years as a minimum up to hundreds and even more than a 
thousand years. But the spectroscope reveals the fact that the 
many thousands of telescopic binaries form only a very small 
part of the binary systems in existence. The overwhelming 



EVOLUTION OF THE STAR SYSTEM 123 

importance of this discovery is, that it carries the times of 
revolution from the minimum of the telescopic doubles down- 
ward in unbroken series through periods of a few years, to those 
reckoned by months, by days, and even by hours. And with 
this reduction of period there necessarily follows a correspond- 
ing reduction of distance, so that sometimes the two stars must 
be in contact, and thus the actual birth or origin of a double 
star has been observed to occur, even though not actually seen. 
This mode of origin was indeed anticipated by Dr. Lee of 
Chicago in 1892, and it has been confirmed by observation in 
the short space of ten years. 

In a remarkable communication to Nature (September 12th, 
1901), Mr. Alexander W. Roberts of Lovedale, South Africa, 
gives some of the main results of this branch of enquiry. Of 
course all the variable stars are to be found among the spectro- 
scopic binaries. They consist of that portion of the class in 
which the plane of the orbit is directed towards us, so that 
during their revolution one of the pair either wholly or partially 
eclipses the other. In some of these cases there are irreg- 
ularities, such as double maxima and minima of unequal lengths, 
which may be due to triple systems or to other causes not yet 
explained, but as they all have short periods and always appear 
as one star in the most powerful telescopes, they form a special 
division of the spectroscopic binary systems. 

There are known at present twenty-two variables of the Algol 
type, that is, stars having each a dark companion very close 
to it which obscures it either wholly or partially during every 
revolution. In these cases the density of the systems can be 



124 MAN'S PLACE IN THE UNIVERSE 

approximately determined, and they are found to be, on the 
average, only one-fifth that of water, or one-eighth that of our 
sun. But as many of them are as large as our sun, or even 
considerably larger, it is evident that they must be wholly 
gaseous, and, even if very hot, of a less complex constitution 
than our luminary. Mr. A. W. Roberts tells us that five out 
of these twenty-two variables revolve in absolute contact, form- 
ing systems of the shape of a dumb-bell. The periods vary 
from twelve days to less than nine hours ; and, starting from 
these, we now have a continuous series of lengthening periods 
up to the twin stars of Castor, which require more than a thou- 
sand years to complete their revolution. 

During his observations of the above five stars, Mr. Roberts 
states that one, X Carina?, was found to have parted company, 
so that instead of being actually united to its companion the 
two are now at a distance apart equal to one-tenth of their 
diameters, and he may thus be said to have been almost a witness 
of the birth of a stellar system. 

A year later we find the record (in Knowledge, October, 1902) 
of Professor Campbell's researches at the Lick Observatory. 
He states that, out of 350 stars observed spectroscopically, one 
in eight is a spectroscopic binary ; and so impressed is he with 
their abundance that, as accuracy of measurement increases, he 
believes that the star that is not a spectroscopic binary will 
prove to be the rare exception! Professor G. Darwin had 
already shown that the " dumb-bell " was a figure of equilibrium 
in a rotating mass of fluid; and we now find proofs that such 
figures exist, and that they form the starting point for the 



EVOLUTION OF THE STAR SYSTEM 125 

enormous and ever-increasing quantities of spectroscopic binary 
star-systems that are now known. The origin of these binary 
stars is also of especial interest as giving support to Professor 
Darwin's well-known explanation of the origin of the moon by 
disruption from the earth, owing to the very rapid rotation of 
the parent planet. It now appears that suns often subdivide 
in the same manner, but, owing perhaps to their intensely 
heated gaseous state, they seem usually to form nearly equal 
globes. The evolution of this special form of star-system is 
therefore now an observed fact ; though it by no means follows 
that all double stars have had the same mode of origin. 

CLUSTERS OF STARS AND VARIABLES 

The clusters of stars, which are tolerably abundant in the 
heavens and offer so many strange and beautiful forms to the 
telescopist, are yet among the most puzzling phenomena the 
philosophic astronomer has to deal with. 

Many of these clusters which are not very crowded and of 
irregular forms, strongly suggest an origin from the equally ir- 
regular and fantastic forms of nebulae by a process of aggrega- 
tion like that which Dr. Roberts describes as developing within 
the spiral nebulae. But the dense globular clusters which form 
such beautiful telescopic objects, and in some of which more 
than six thousand stars have been counted besides considerable 
numbers so crowded in the centre as to be uncountable, are more 
difficult to explain. One of the problems suggested by these 
clusters is as to their stability. Professor Simon Newcomb re- 



126 MAN'S PLACE IN THE UNIVERSE 

marks on this point as follows : " Where thousand of stars are 
condensed into a space so small, what prevents them from all 
falling together into one confused mass ? Are they really doing 
so, and will they ultimately form a single body? These are 
questions which can be satisfactorily answered only by centuries 
of observation ; they must therefore be left to the astronomers 
of the future." 

There are, however, some remarkable features in these clus- 
ters which afford possible indications of their origin and 
essential constitution. When closely examined most of them 
are seen to be less regular than they at first appear. Vacant 
spaces can be noted in them; even rifts of definite forms. In 
some there is a radiated structure; in others there are curved 
appendages ; while some have fainter centres. These features 
are so exactly like what are found, in a more pronounced form, 
in the larger nebulas, that we can hardly help thinking that in 
these clusters we have the result of the condensation of very 
large nebulae, which have first aggregated towards numerous 
centres, while these agglomerations have been slowly drawn 
towards the common centre of gravity of the whole mass. It is 
suggestive of this origin that while the smaller telescopic nebulae 
are far removed from the Milky Way, the larger ones are most 
abundant near its borders; while the star-clusters are excess- 
ively abundant on and near the Milky Way, but very scarce 
elsewhere, except in or near vast nebulas like the Magellanic 
Clouds. We thus see that the two phenomena may be comple- 
mentary to each other, the condensation of nebulae having 
gone on most rapidly where material was most abundant, re- 






EVOLUTION OF THE STAR SYSTEM 127 

suiting in numerous star-clusters where there are now few 
nebulae. 

There is one striking feature of the globular clusters which 
calls for notice; the presence in some of them of enormous 
quantities of variable stars, while in others few or none can be 
found. The Harvard Observatory has for several years de- 
voted much time to this class of observations, and the results 
are given in Professor Newcomb's recent volume on The 
Stars. It appears that twenty-three clusters have been ob- 
served spectroscopically, the number of stars examined in each 
cluster varying from 145 up to 3000, the total number of stars 
thus minutely tested being 19,050. Out of this total number 
509 were found to be variable; but the curious fact is, the 
extreme divergence in the proportion of variables to the whole 
number examined in the several clusters. In two clusters 
though 1279 stars were examined not a single variable was 
found. In three others the proportion was from one in 1050 
to one in 500. Five more ranged up to one in 100, and the re- 
mainder showed from that proportion up to one in seven, 900 
stars being examined in the last-mentioned cluster of which 132 
were variable ! 

When we consider that variable stars form only a portion, 
and necessarily a very small proportion of binary systems of 
stars, it follows that in all the clusters which show a large 
proportion of variables, a very much larger proportion — in 
some cases perhaps all, must be double or multiple stars re- 
volving round each other. With this remarkable evidence, in 
addition to that adduced for the prevalence of double stars and 



128 MAN'S PLACE IN THE UNIVERSE 

variables among the stars in general, we can understand Pro- 
fessor Newcomb adding his testimony to that of Professor 
Campbell already quoted, that : " It is probable that among 
the stars in general, single stars are the exception rather than 
the rule. If such be the case, the rule should hold yet more 
strongly among the stars of a condensed cluster." 

THE EVOLUTION OF THE STARS 

So long as astronomers were limited to the use of the telescope 
only, or even the still greater powers of the photographic plate, 
nothing could be learnt of the actual constitution of the stars 
or of the process of their evolution. Their apparent magni- 
tudes, their movements, and even the distances of a few could 
be determined; while the diversity of their colours offered the 
only clue (a very imperfect one) even to their temperature. 
But the discovery of spectrum-analysis has furnished the means 
of obtaining some definite knowledge of the physics and chem- 
istry of the stars, and has thus established a new branch of 
science — Astrophysics — which has already attained large pro- 
portions, and which furnishes the materials for a periodical and 
some important volumes. This branch of the subject is very 
complex, and as it is not directly connected with our present 
inquiry, it is only referred to again in order to introduce such 
of its results as bear upon the question of the classification and 
evolution of the stars. 

By a long series of laboratory experiments it has been shown 
that numerous changes occur in the spectra of the elements when 



EVOLUTION OF THE STAR SYSTEM 129 

subjected to different temperatures, ranging upwards to the 
highest attainable by means of a battery producing an electric 
spark several feet long. These changes are not in the relative 
position of the bands or dark lines, but in their number, breadth, 
and intensity. Other changes are due to the density of the 
medium in which the elements are heated, and to their chemical 
condition as to purity; and from these various modifications 
and their comparison with the solar spectrum and those of its 
appendages, it has become possible to determine, from the 
spectrum of a star, not only its temperature as compared with 
that of the electric spark and of the sun, but also its place in a 
developmental series. 

The first general result obtained by this research is, that the 
bluish white or pure white stars, having a spectrum extending 
far towards the violet end, and which exhibits the coloured bands 
of gases only, usually hydrogen and helium, are the hottest. 
Next come those with a shorter spectrum not extending so far 
towards the violet end, and whose light is therefore more yellow 
in tint. To this group our sun belongs ; and they are all 
characterised like it by dark lines due to absorption, and by the 
presence of metals, especially iron, in a gaseous state. The 
third group have the shortest spectra and are of a red colour, 
while their spectra contain lines denoting the presence of carbon. 
These three groups are often spoken of as " gaseous stars," 
" metallic stars," and " carbon stars." Other astronomers call 
the first group " Sirian stars," because Sirius, though not the 
hottest, is a characteristic type ; the second being termed " solar 
stars " ; others again speak of them as stars of Class L, Class 



130 MAN'S PLACE IN THE UNIVERSE 

II., etc., according to the system of classification they have 
adopted. It was soon perceived, however, that neither the 
colour nor the temperature of stars gave much information as 
to their nature and state of development, because, unless we 
supposed the stars to begin their lives already intensely hot 
(and all the evidence is against this), there must be a period 
during which heat increases, then one of maximum heat, fol- 
lowed by one of cooling and final loss of light altogether. The 
meteoritic theory of the origin of all luminous bodies in the 
heavens, now very widely adopted, has been used, as we have 
seen, to explain the development of stars from nebulae, and its 
chief exponent in England, Sir Norman Lockyer, has pro- 
pounded a complete scheme of stellar evolution and decay which 
may be here briefly outlined : 

Beginning with nebulas, we pass on to stars having banded 
or fluted spectra, indicating comparatively low temperatures 
and showing bands or lines of iron, manganese, calcium, and 
other metals. They are more or less red in colour, Ant ares in 
the Scorpion being one of the most brilliant red stars known. 
These stars are supposed to be in the process of aggregation, 
to be continually increasing in size and heat, and thus to be 
subject to great disturbances. Alpha Cygni has a similar 
spectrum but with more hydrogen, and is much hotter. The 
increase of heat goes on through Rigel and Beta Crucis, in 
which we find mainly hydrogen, helium, oxygen, nitrogen, and 
also carbon, but only faint traces of metals. Reaching the 
hottest of all — Epsilon Orionis and two stars in Argo — 
hydrogen is predominant, with traces of a few metals and 



EVOLUTION OF THE STAR SYSTEM 131 

carbon. The cooling series is indicated by thicker lines of 
hydrogen and thinner lines of the metallic elements, through 
Sirius, to Arcturus and our sun, thence to 19 Piscium, which 
shows chiefly flutings of carbon, with a few faint metallic 
lines. The process of further cooling brings us to the dark 
stars. 

We have here a complete scheme of evolution, carrying us 
from those ill-defined but enormously diffused masses of gas and 
cosmic dust we know as nebulae, through planetary nebulae, 
nebulous stars, variable and double stars, to red and white stars 
and on to those exhibiting the most intense blue-white lustre. 
We must remember, however, that the most brilliant of these 
stars, showing a gaseous spectrum and forming the culminating 
point of the ascending series, are not necessarily hotter, or even 
so hot as some of those far down on the descending scale ; since 
it is one of the apparent paradoxes of physics that a body may 
become hotter during the very process of contraction through 
loss of heat. The reason is that by cooling it contracts and 
thus becomes denser, that a portion of its mass falls towards its 
centre, and in doing so produces an amount of heat which, 
though absolutely less than the heat lost in cooling, will under 
certain conditions cause the reduced surface to become hotter. 
The essential point is, that the body in question must be 
wholly gaseous, allowing of free circulation from surface 
to centre. The law, as given by Professor S. Newcomb, is as 
follows : 

" When a spherical mass of incandescent gas contracts 
through the loss of its heat by radiation into space, its tempera- 



132 MAN'S PLACE IN THE UNIVERSE 

ture continually becomes higher as long as the gaseous condition 

is retained." 

To put it in another way, if the compression was caused by 
external force and no heat was lost, the globe would get hotter 
by a calculable amount for each unit of contraction. But the 
heat lost in causing a similar amount of contraction is so little 
more than the increase of heat produced by contraction, that 
the slightly diminished total heat in a smaller bulk causes the 
temperature of the mass to increase. 

But if, as there is reason to believe, the various types of stars 
differ also in chemical constitution, some consisting mainly of 
the more permanent gases, while in others the various metallic 
and non-metallic elements are present in very different propor- 
tions, there should really be a classification by constitution as 
well as by temperature, and the course of evolution of the 
differently constituted groups may be to some extent dis- 
similar. 

With this limitation, the process of evolution and decay of 
suns through a cycle of increasing and decreasing temperature, 
as suggested by Sir Norman Lockyer, is clear and suggestive. 
During the ascending series the star is growing both in mass and 
heat, by the continual accretion of meteoritic matter either 
drawn to it by gravitation or falling towards it through the 
proper motions of independent masses. This goes on till all the 
matter for some distance around the star has been utilised, and 
a, maximum of size, heat, and brilliancy attained. Then the 
loss of heat by radiation 1 is no longer compensated by the influx 
of fresh matter, and a slow contraction occurs accompanied by 



EVOLUTION OF THE STAR SYSTEM 133 

a slightly increased temperature. But owing to the more stable 
conditions continuous envelopes of metals in the gaseous state 
are formed, which check the loss of heat and reduce the brilliancy 
of colour; whence it follows that bodies like our sun may be 
really hotter than the most brilliant white stars, though not 
giving out quite so much heat. The loss of heat is therefore 
reduced; and this may serve to account for the undoubted fact 
that during the enormous epochs of geological time there has 
been very little diminution in the amount of heat we have 
received from the sun. 

On the general question of the meteoritic hypothesis one of 
our first mathematicians, Professor George Darwin, has thus 
expressed his views : " The conception of the growth of the 
planetary bodies by the aggregation of meteorites is a good 
one, and perhaps seems more probable than the hypothesis that 
the whole solar system was gaseous." I may add, that one of 
the chief objections made to it, that meteorites are too complex 
to be supposed to be the primitive matter out of which suns and 
worlds have been made, does not seem to me valid. The primi- 
tive matter, whatever it was, may have been used up again and 
again, and if collisions of large solid globes ever occur — and 
it is assumed by most astronomers that they must sometimes 
occur — then meteoric particles of all sizes would be produced 
which might exhibit any complexity of mineral constitution. 
The material universe has probably been in existence long 
enough for all the primitive elements to have been again and 
again combined into the minerals found upon the earth and 
many others. It cannot be too often repeated that no explana- 



134 MAN'S PLACE IN THE UNIVERSE 

tion — no theory — can ever take us to the beginning of things, 
but only one or two steps at a time into the dim past, which may 
enable us to comprehend, however imperfectly, the processes by 
which the world, or the universe, as it is, has been developed oui 
of some earlier and simpler condition. 



CHAPTER VII 



ARE THE STARS INFINITE IN NUMBER? 



Most of the critics of my first short discussion of this subject 
laid great stress upon the impossibility of proving that the 
universe, a part of which we see, is not infinite; and a well- 
known astronomer declared that unless it can be demonstrated 
that our universe is finite the entire argument founded upon 
our position within it falls to the ground. I had laid myself 
open to this objection by rather incautiously admitting that if 
the preponderance of evidence pointed in this direction any 
inquiry as to our place in the universe would be useless, because 
as regards infinity there can be no difference of position. But 
this statement is by no means exact, and even in an infinite uni- 
verse of matter containing an infinite number of stars, such as 
those we see, there might well be such infinite diversities of dis- 
tribution and arrangement as would give to certain positions 
all the advantages which I submit we actually possess. Sup- 
posing, for example, that beyond the vast ring of the Milky 
Way the stars rapidly decrease in number in all directions for a 
distance of a hundred or a thousand times the diameter of that 
ring, and that then for an equal distance they slowly increase 
again and become aggregated into systems or universes totally 
distinct from ours in form and structure, and so remote that 

135 



136 MAN'S PLACE IN THE UNIVERSE 

they can influence us in no way whatever. Then, I maintain, 
our position within our own stellar universe might have exactly 
the same importance, and be equally suggestive, as if ours were 
the only material universe in existence — as if the apparent 
diminution in the number of stars (which is an observed fact) 
indicated a continuous diminution, leading at some unknown 
distance to entire absence of luminous — that is, of active, 
energy-emitting aggregations of matter. 1 As to whether there 
are such other material universes or not I offer no opinion, and 
have no belief one way or the other. I consider all speculations 
as to what may or may not exist in infinite space to be utterly 
valueless. I have limited my inquiries strictly to the evidence 
accumulated by modern astronomers, and to direct inferences 
and logical deductions from that evidence. Yet, to my great 
surprise, my chief critic declares that " Dr. Wallace's under- 
lying error is, indeed, that he has reasoned from the area which 
we can embrace with our limited perceptions to the infinite be- 
yond our mental or intellectual grasp." I have distinctly not 
done this, but many astronomers have done so. The late 
Richard Proctor not only continually discussed the question of 
infinite matter as well as infinite space, but also argued, from 
the supposed attributes of the Deity, for the necessity of hold- 
ing this material universe to be infinite, and the last chapter of 
his Other Worlds than Ours is mainly devoted to such specula- 
tions. In a later work, Our Place among Infinities, he says that 
" the teachings of science bring us into the presence of the 

1 In a letter to Knowledge, June, 1903, Mr. W. H. T. Monck puts the same 
point in a mathematical form. 



ARE THE STARS INFINITE IN NUMBER? 137 

unquestionable infinities ofj time and of space, and the presum- 
able infinities of matter and of operation — hence therefore into 
the presence of infinity of energy. But science teaches us noth- 
ing about these infinities as such. They remain none the less 
inconceivable, however clearly we may be taught to recognise 
their reality." All this is very reasonable, and the last sentence 
is particularly important. Nevertheless, many writers allow 
their reasonings from facts to be influenced by these ideas of 
infinity. In Proctor's posthumous work, Old and New Astron- 
omy, the late Mr. Ranyard, who edited it, writes: " If we reject 
as abhorrent to our minds the supposition that the universe is 
not infinite, we are thrown back on one of two alternatives — 
either the ether which transmits the light of the stars to us is 
not perfectly elastic, or a large proportion of the light of the 
stars is obliterated by dark bodies." Here we have a well- 
informed astronomer allowing his abhorrence of the idea of a 
finite universe to affect his reasoning on the actual phenomena 
we can observe — doing in fact exactly what my critic errone- 
ously accuses me of doing. But setting aside all ideas and 
prepossessions of the kind here indicated, let us see what are 
the actual facts revealed by the best instruments of modern as- 
tronomy, and what are the natural and logical inferences from 
those facts. 

AEE THE STARS INFINITE IN NUMBER? 

The views of those astronomers who have paid attention to this 
subject are, on the whole, in favour of the view that the stellar 
universe is limited in extent and the stars therefore limited in 



138 MAN'S PLACE IN THE UNIVERSE 

number. A few quotations will best exhibit their opinions on 
this question, with some of the facts and observations on which 
they are founded. 

Miss A. M. Clerke, in her admirable volume, The System of 
the Stars, says : " The sidereal world presents us, to all appear- 
ance, with a finite system. . . . The probability amounts almost 
to certainty that star-strewn space is of measurable dimensions. 
For from innumerable stars a limitless sum-total of radiations 
should be derived, by which darkness would be banished from our 
skies ; and the ' intense inane,' glowing with the mingled beams 
of suns individually indistinguishable, would bewilder our 
feeble senses with itd monotonous splendour. . . . Unless, that 
is to say, light suffer some degree of enfeeblement in space. . . . 
But there is not a particle of evidence that any such toll is 
exacted; contrary indications are strong; and the assertion 
that its payment is inevitable depends upon analogies which 
may be wholly visionary. We are then, for the present, entitled 
to disregard the problematical effect of a more than dubious 
cause." 

Professor Simon Newcomb, one of the first of American 
mathematicians and astronomers, arrives at a similar conclusion 
in his most recent volume, The Stars (1902). He says, in his 
conclusions at the end of the work : " That collection of stars 
which we call the universe is limited in extent. The smallest 
stars that we see with the most powerful telescopes are not, 
for the most part, more distant than those a grade brighter, 
but are mostly stars of less luminosity situate in the same 
regions " (p. 319). And on page 229 of the same work he 



ARE THE STARS INFINITE IN NUMBER? 139 

gives reasons for this conclusion, as follows : " There is a law 
of optics which throws some light on the question. Suppose 
the stars to be scattered through infinite space so that every 
great portion of space is, in the general average, equally rich 
in stars. Then at some great distance we describe a sphere 
having its centre in our sun. Outside this sphere describe 
another one of a greater radius, and beyond this other spheres 
at equal distances apart indefinitely. Thus we shall have an 
endless succession of spherical shells, each of the same thickness. 
The volume of each of these shells will be nearly proportional to 
the squares of the diameters of the spheres which bound it. 
Hence each of the regions will contain a number of stars in- 
creasing as the square of the radius of the region. Since the 
amount of light we receive from each star is as the inverse 
square of its distance, it follows that the sum total of the light 
received from each of these spherical shells will be equal. Thus 
as we add sphere after sphere we add equal amounts of light 
without limit. The result would be that if the system of stars 
extended out indefinitely the whole heavens would be filled with 
a blaze of light as bright as the sun." 

But the whole light given us by the stars is variously esti- 
mated at from one-fortieth to one-twentieth or, as an extreme 
limit, to one-tenth of moonlight, while the sun gives as much 
light as 300,000 full moons, so that starlight is only equivalent 
at a fair estimate to the six-millionth part of sunlight. Keep- 
ing this in mind, the possible causes of the extinction of almost 
the whole of the light of the stars (if they are infinite in number 
and distributed, on the average, as thickly beyond the Milky 



140 MAN'S PLACE IN THE UNIVERSE 

Way as they are up to its outer boundary) are absurdly inade- 
quate. These causes are (1) the loss of light in passing 
through the ether, and (2) the stoppage of light by dark stars 
or diffused meteoritic dust. As to the first, it is generally ad- 
mitted that there is not a particle of evidence of its existence. 
There is, however, some distinct evidence that, if it exists, it is 
so very small in amount that it would not produce a perceptible 
effect for any distances less remote than hundreds or perhaps 
thousands of times as far as the furthest limits of the Milky 
Way are from us. This is indicated by the fact that the 
brightest stars are not always, or even generally, the nearest to 
us, as is shown both by their small proper motions and the 
absence of measurable parallax. Mr. Gore states that out of 
twenty-five stars, with proper motions of more than two seconds 
annually, only two are above the third magnitude. Many first 
magnitude stars, including Canopus, the second brightest star 
in the heavens, are so remote that no parallax can be found, 
notwithstanding repeated efforts. They must therefore be 
much further off than many small and telescopic stars, and 
perhaps as far as the Milky Way, in which so many brilliant 
stars are found; whereas if any considerable amount of light 
were lost in passing that distance we should find but few stars 
of the first two or three magnitudes that were very remote from 
us. Of the twenty-three stars of the first magnitude, only ten 
have been found to have parallaxes of more than one-twentieth 
of a second, while five range from that small amount down to 
one or two hundredths of a second, and there are two with no 
ascertainable parallax. Again, there are 309 stars brighter 



ARE THE STARS INFINITE IN NUMBER? 141 

than magnitude 3.5, yet only thirty-one of these have proper 
motions of more than 100" a century, and of these only eighteen 
have parallaxes of more than one-twentieth of a second. These 
figures are from tables given in Professor Newcomb's book, and 
they have very great significance, since they indicate that the 
brightest stars are not the nearest to us. More than this, they 
show that out of the seventy-two stars whose distance has been 
measured with some approach to certainty, only twenty-three 
(having a parallax of more than one-fiftieth of a second) are 
of greater magnitudes than 3.5, while no fewer than forty -nine 
are smaller stars down to the eighth or ninth magnitude, and 
these are on the average much nearer to us than the brighter 
stars ! 

Taking the whole of the stars whose parallaxes are given by 
Professor Newcomb we find that the average parallax of the 
thirty-one bright stars (from 3.5 magnitude up to Sirius) is 
0.11 seconds ; while that of the forty-one stars below 3.5 magni- 
tude down to about 9.5, is 0.21 seconds, showing that they are, 
on the average, only half as far from us as the brighter stars. 
The same conclusion was reached by Mr. Thomas Lewis of the 
Greenwich Observatory in 1895, namely, that the stars from 
2.70 magnitude down to about 8.40 magnitude have, on the 
average, double the parallaxes of the brighter stars. This 
very curious and unexpected fact, however it may be accounted 
for, is directly opposed to the idea of there being any loss of 
light by the more distant as compared with the nearer stars ; for 
if there should be such a loss it would render the above phenom- 
enon still more difficult of explanation, because it would tend 



142 MAN'S PLACE IN THE UNIVERSE 

to exaggerate it. The bright stars being on the whole farther 
away from us than the less bright down to the eighth and ninth 
magnitudes, it follows, if there is any loss of light, that the 
bright stars are really brighter than they appear to us, because, 
owing to their enormous distance some of their light has been 
lost before it reached us. Of course it may be said that this 
does not demonstrate that no light is lost in passing through 
space ; but, on the other hand, it is exactly the opposite of what 
we should expect if the more distant stars were perceptibly 
dimmed by this cause, and it may be considered to prove that if 
there is any loss it is exceedingly small, and will not affect the 
question of the limits of our stellar system, which is all that we 
are dealing with. 

This remarkable fact of the enormous remoteness of the 
majority of the brighter stars is equally effective as an argu- 
ment against the loss of light by dark stars or cosmic dust, 
because, if the light is not appreciably diminished for stars 
which have less than the fiftieth of a second of parallax, it can- 
not greatly interfere with our estimates of the limits of our 
universe. 

Both Mr. E. W. Maunder of the Greenwich Observatory and 
Professor W. W. Turner of Oxford lay great stress on these 
dark bodies, and the former quotes Sir Robert Ball as saying, 
" the dark stars are incomparably more numerous than those 
that we can see . . . and to attempt to number the stars of our 
universe by those whose transitory brightness we can perceive 
would be like estimating the number of horseshoes in England 
by those which are red-hot." But the proportion of dark stars 



ARE THE STARS INFINITE IN NUMBER? 143 

(or nebulae) to bright ones cannot be determined a priori, since 
it must depend upon the causes that heat the stars, and how 
frequently those causes come into action as compared with the 
life of a bright star. We do know, both from the stability of 
the light of the stars during the historic period, and much more 
precisely by the enormous epochs during which our sun has sup- 
ported life upon this earth — yet which must have been " incom- 
parably " less than its whole existence as a light-giver — that the 
life of most stars must be counted by hundreds or perhaps by 
thousands of millions of years. But we have no knowledge 
whatever of the rate at which true stars are born. The so- 
called " new stars " which occasionally appear, evidently 
belong to a different category. They blaze out suddenly 
and almost as suddenly fade away into obscurity or total 
invisibility. But the true stars probably go through their 
stages of origin, growth, maturity, and decay, with 
extreme slowness, so that it is not as yet possible for us to 
determine by observation when they are born or when they die. 
In this respect they correspond to species in the organic world. 
They would probably first be known to us as stars or minute 
nebulas at the extreme limit of telescopic vision or of photo- 
graphic sensitiveness, and the growth of their luminosity might 
be so gradual as to require hundreds, perhaps thousands of 
years to be distinctly recognisable. Hence the argument 
derived from the fact that we have never witnessed the birth of 
a true permanent star, and that, therefore, such occurrences are 
very rare, is valueless. New stars may arise every year or every 
day without our recognising them; and if this is the case the 



144 MAN'S PLACE IN THE UNIVERSE 

reservoir of dark bodies, whether in the form of large masses or 
of clouds of cosmic dust, so far from being incomparably 
greater than the whole of the visible stars and nebulae, may quite 
possibly be only equal to it, or at most a few times greater ; and 
in that case, considering the enormous distances that separate 
the stars (or star-systems) from each other, they would have 
no appreciable effect in shutting out from our view any con- 
siderable proportion of the luminous bodies constituting our 
stellar universe. It follows, that Professor Newcomb's argu- 
ment as to the very small total light given by the stars, has not 
been even weakened by any of the facts or arguments adduced 
against it. 

Mr. W. H. T. Monck, in a letter to Knowledge (May, 1903), 
puts the case very strongly so as to support my view. He 
says : " The highest estimate that I have seen of the total light 
of the full moon is to^.to(T °f that of the sun. Suppose 
that the dark bodies were a hundred and fifty thousand times 
as numerous as the bright ones. Then the whole sky ought to 
be as bright as the illuminated portion of the moon. Everyone 
knows that this is not so. But it is said that the stars, though 
infinite, may only extend to infinity in particular directions, e. g., 
in that of the Galaxy. Be it so. Where, in the very brightest 
portion of the Galaxy, will we find a part equal in angular mag- 
nitude to the moon which affords us the same quantity of light ? 
In the very brightest spot, the light probably does not amount 
to one hundredth part that of the full moon." It follows that, 
even if dark stars were fifteen million times as numerous as the 
bright ones, Professor Newcomb's argument would still apply 



ARE THE STARS INFINITE IN NUMBER? 145 

against an infinite universe of stars of the same average density 
as the portion we see. 

TELESCOPIC EVIDENCE AS TO THE LIMITS OF 
THE STAR SYSTEM 

Throughout the earlier portion of the nineteenth century every 
increase of power and of light-giving qualities of telescopes 
added so greatly to the number of the stars which became visible, 
that it was generally assumed that this increase would go on 
indefinitely, and that the stars were really infinite in number and 
could not be exhausted. But of late years it has been found 
that the increase in the number of stars visible in the larger 
telescopes was not so great as might be expected, while in many 
parts of the heavens a longer exposure of the photographic 
plate adds comparatively little to the number of stars obtained 
by a shorter exposure with the same instrument. 

Mr. J. E. Gore's testimony on this point is very clear. He 
says : " Those who do not give the subj ect sufficient considera- 
tion, seem to think that the number of the stars is practically 
infinite, or at least, that the number is so great that it cannot be 
estimated. But this idea is totally incorrect, and due to com- 
plete ignorance of telescopic revelations. It is certainly true 
that, to a certain extent, the larger the telescope used in the 
examination of the heavens, the more the number of the stars 
seems to increase ; but we now know that there is a limit to this 
increase of telescopic vision. And the evidence clearly shows 
that we are rapidly approaching this limit. Although the num- 



146 MAN'S PLACE IN THE UNIVERSE 

ber of stars visible in the Pleiades rapidly increases at first with 
increase in the size of the telescope used, and although photog- 
raphy has still further increased the number of stars in this 
remarkable cluster, it has recently been found that an increased 
length of exposure — beyond three hours — adds very few stars 
to the number visible on the photograph taken at the Paris 
Observatory in 1885, on which over two thousand stars can be 
counted. Even with this great number on so small an area of 
the heavens, comparatively large vacant places are visible 
between the stars, and a glance at the original photograph is 
sufficient to show that there would be ample room for many 
times the number actually visible. I find that if the whole 
heavens were as rich in stars as the Pleiades, there would be only 
thirty-three millions in both hemispheres." 

Again, referring to the fact that Celoria, with a telescope 
showing stars down to the eleventh magnitude, could see almost 
exactly the same number of stars near the north pole of the 
Galaxy as Sir William Herschel found with his much larger and 
more powerful telescope, he remarks : " Their absence, therefore, 
seems certain proof that very faint stars do not exist in that 
direction, and that here, at least, the sidereal universe is limited 
in extent." 

Sir John Herschel notes the same phenomena, stating that 
even in the Milky Way there are found " spaces absolutely dark 
and completely void of any star, even of the smallest telescopic 
magnitude " ; while in other parts " extremely minute stars, 
though never altogether wanting, occur in numbers so moderate 
as to lead us irresistibly to the conclusion that in these regions 






ARE THE STARS INFINITE IN NUMBER? 147 

we see fairly through the starry stratum, since it is impossible 
otherwise (supposing their light not intercepted) that the num- 
bers of the smaller magnitudes should not go on continually 
increasing ad infinitum. In such cases, moreover, the ground 
of the heavens, as seen between the stars, is for the most part 
perfectly dark, which again would not be the case if innumerable 
multitudes of stars, too minute to be individually discernible, 
existed beyond." And again he sums up as follows . " Through- 
out by far the larger portion of the extent of the Milky Way in 
both hemispheres, the general blackness of the ground of the 
heavens, on which its stars are projected, and the absence of 
that innumerable multitude and excessive crowding of the 
smallest visible magnitudes, and of glare produced by the ag- 
gregate light of multitudes too small to affect the eye singly, 
which the contrary supposition would appear to necessitate, 
must, we think, be considered unequivocal indications that its 
dimensions in directions where these conditions obtain, are 
not only not infinite, but that the space-penetrating power 
of our telescopes suffices fairly to pierce through and beyond 
it." x 

The expression of opinion by the astronomer who, probably 
beyond any now living, was the most competent authority on 
this question, to which he devoted a long life of observation and 
study extending over the whole heavens, cannot be lightly set 
aside by the opinions or conjectures of those who seem to assume 
that we must believe in an infinity of stars if the contrary can- 

1 Outlines of Astronomy (last edition), pp. 578-9. In the passages quoted 
the italics are Sir John HerschePs. 



148 MAN'S PLACE IN THE UNIVERSE 

not be absolutely proved. But as not a particle of evidence 
can be adduced to prove infinity, and as all the facts and indi- 
cations point, as here shown, in a directly opposite direction, 
we must, if we are to trust to evidence at all in this matter, 
arrive at the conclusion that the universe of stars is limited 
in extent. 

Dr. Isaac Roberts gives similar evidence as regards the use 
of photographic plates. He writes : " Eleven years ago photo- 
graphs of the Great Nebula in Andromeda were taken with the 
20-inch reflector, and exposures of the plates during intervals 
up to four hours ; and upon some of them were depicted stars 
to the faintness of 17th to 18th magnitude, and nebulosity to 
an equal degree of faintness. The films of the plates obtain- 
able in those days were less sensitive than those which have been 
available during the past five years, and during this period 
photographs of the nebula with exposures up to four hours have 
been taken with the SO-inch reflector. No extensions of the 
nebulosity, however, nor increase in the number of the stars can 
be seen on the later rapid plates than were depicted upon the 
earlier slower ones, though the star-images and the nebulosity 
have greater density on the later plates." 

Exactly similar facts are recorded in the cases of the Great 
Nebula in Orion, and the group of the Pleiades. In the case of 
the Milky Way in Cygnus photographs have been taken with 
the same instrument, but with exposures varying from one hour 
to two hours and a half, but no fainter stars could be found on 
one than on the other; and this fact has been confirmed by 
similar photographs of other areas in the sky. 



ARE THE STARS INFINITE IN NUMBER? 149 

THE LAW OF DIMINISHING NUMBERS OF STARS 

We will now consider another kind of evidence equally weighty 
with the two already adduced. This is what may be termed the 
law of diminishing numbers beyond a certain magnitude, as ob- 
served by larger and larger telescopes. 

For some years past star-magnitudes have been determined 
very accurately by means of careful photometric comparisons. 
Down to the sixth magnitude stars are visible to the naked eye, 
and are hence termed lucid stars. All fainter stars are tel- 
escopic, and continuing the magnitudes in a series in which the 
difference in luminosity between each successive magnitude is 
equal, the seventeenth magnitude is reached and indicates the 
range of visibility in the largest telescopes now in existence. 
By the scale now used a star of any magnitude gives nearly two 
and a half times as much light as one of the next lower magni- 
tude, and for accurate comparison the apparent brightness of 
each star is given to the tenth of a magnitude which can easily 
be observed. Of course, owing to differences in the colour of 
stars, these determinations cannot be made with perfect ac- 
curacy, but no important error is due to this cause. According 
to this scale a sixth magnitude star gives about one-hundredth 
part of the light of an average first magnitude star. Sirius is 
so exceptionally bright that it gives nine times as much light as 
a standard or average first magnitude star. 

Now it is found that from the first to the sixth magnitude the 
stars increase in number at the rate of about three and a half 
times those of the preceding magnitudes. The total number of 



160 MAN'S PLACE IN THE UNIVERSE 

stars down to the sixth magnitude is given by Professor New- 
comb as 7647. For higher magnitudes the numbers are so 
great that precision and uniformity are more difficult of at- 
tainment ; yet there is a wonderful continuance of the same law 
of increase down to the tenth magnitude, which is estimated to 
include 2,311,000 stars, thus conforming very nearly with the 
ratio of 3.5 as determined by the lucid stars. 

But when we pass beyond the tenth magnitude to those vast 
numbers of faint stars only to be seen in the best or the largest 
telescopes, there appears to be a sudden change in the ratio 
of increased numbers per magnitude. The numbers of these 
stars are so great that it is impossible to count the whole as with 
the higher magnitude stars, but numerous counts have been 
made by many astronomers, in small measured areas in different 
parts of the heavens, so that a fair average has been obtained, 
and it is possible to make a near approximation to the total 
number visible down to the seventeenth magnitude. The esti- 
mate of these by astronomers who have made a special study of 
this subject is, that the total number of visible stars does not 
exceed one hundred millions. 1 

But if we take the number of stars down to the ninth magni- 
tude, which are known with considerable accuracy, and find the 
numbers in each succeeding magnitude down to the seventeenth, 
according to the same ratio of increase which has been found to 
correspond very nearly in the case of the higher magnitudes, 
Mr. J. E. Gore finds that the total number should be about 1400 
millions. Of course neither of these estimates makes any pre- 
1 Mr. J. E. Gore in Concise Knowledge Astronomy, pp. 541-2. 



ARE THE STARS INFINITE IN NUMBER: 151 

tence to exact accuracy, but they are founded on all the facts 
at present available, and are generally accepted by astronomers 
as being the nearest approach that can be made to the true 
numbers. The discrepancy is, however, so enormous that 
probably no careful observer of the heavens with very large 
telescopes doubts that there is a very real and very rapid 
diminution in the numbers of the fainter as compared with the 
brighter stars. 

There is, however, yet one more indication of the decreasing 
numbers of the faint telescopic stars, which is almost conclu- 
sive on this question, and, so far as I am aware, has not yet 
been used in this relation. I will therefore briefly state it. 

THE LIGHT RATIO AS INDICATING THE NUMBER 
OF FAINT STARS 

Professor Newcomb points out a remarkable result depending 
on the fact that, while the average light of successively lower 
magnitudes diminishes in a ratio of 2.5, their numbers increase 
at nearly a ratio of 3.5. From this it follows that, so long as 
this law of increase continues, the total of the star-light 
goes on increasing by about forty per cent, for each suc- 
cessive magnitude, and he gives the following table to illus- 
trate it: 



152 



MAN'S PLACE IN THE UNIVERSE 



Mag. 1 



3 
4 
5 
6 
7 
8 
9 
10 



Total Light 


= 1 


M 




= 1.4 


« 




= 2.0 


« 




= 2.8 


a 




= 4.0 


u 




= 5.7 


« 




= 8.0 


it 




= 11.3 


« 




= 16.0 


M 


To 


= 22.6 




tal, 74.8 



Thus the total amount of the light given by all stars down 
to the tenth magnitude is seventy-four times as great as that 
from the few first magnitude stars. We also see that the light 
given by the stars of any magnitude is twice as much as that of 
stars two magnitudes higher in the scale, so that we can easily 
calculate what additional light we ought to receive from each ad- 
ditional magnitude if they continue to increase in numbers below 
the tenth as they do above that magnitude. Now it has been 
calculated as the result of careful observations, that the total 
light given by stars down to nine and a half magnitude is one- 
eightieth of full moonlight, though some make it much more. 
But if we continue the table of light-ratios from this low start- 
ing-point down to magnitude seventeen and a half, we shall find, 
if the numbers of the stars go on increasing at the same rate 
as before, that the light of all combined should be at least seven 
times as great as moonlight ; whereas the photometric measure- 
ments make it actually about one-twentieth. And as the calcu- 
lation from light-ratios only includes stars just visible in the 



ARE THE STARS INFINITE IN NUMBER? 153 

largest telescopes, and does not include all those proved to exist 
by photography, we have in this case a demonstration that the 
numbers of the stars below the tenth and down to the seven- 
teenth magnitude diminish rapidly. 

We must remember that the minuter telescopic stars prepon- 
derate enormously in and near the Milky Way. At a distance 
from it they diminish rapidly, till near its poles they are almost 
entirely absent. This is shown by the fact (already referred to 
at p. 146) that Professor Celoria, Milan, with a telescope of 
less than three inches aperture, counted as many stars in that 
region as did Herschel with his eighteen-inch reflector. But if 
the stellar universe extends without limit we can hardly suppose 
it to do so in one plane only ; hence the absence of the minuter 
stars and of diffused milky light over the larger part of the 
heavens is now held to prove that the myriads of very minute 
stars in the Milky Way really belong to it, and not to the depths 
of space far beyond. 

It seems to me that here we have a fairly direct proof 
that the stars of our universe are really limited in number. 

There are thus four distinct lines of argument all pointing 
with more or less force to the conclusion that the stellar universe 
we see around us, so far from being infinite, is strictly limited 
in extent and of a definite form and constitution. They may 
be briefly summarised as follows : 

( 1 ) Professor Newcomb shows that, if the stars were infinite 
in number, and if those we see were approximately a fair sample 
of the whole, and further, if there were not sufficient dark bodies 
to shut out almost the whole of their light, then we should 



154 MAN'S PLACE IN THE UNIVERSE 

receive from them an amount of light theoretically greater than 
that of sunlight. I have shown, at some length, that neither 
of these causes of loss of light will account for the enormous 
disproportion between the theoretical and the actual light re- 
ceived from the stars; and therefore Professor Newcomb's 
argument must be held to be a valid one against the infinite 
extent of our universe. Of course, this does not imply that 
there may not be any number of other universes in space, but 
as we know absolutely nothing of them — even whether they are 
material or non-material — all speculation as to their existence 
is worse than useless. 

(2) The next argument depends on the fact that all over the 
heavens, even in the Milky Way itself, there are areas of con- 
siderable extent, besides rifts, lanes, and circular patches, where 
stars are either quite absent or very faint and few in number. 
In many of these areas the largest telescopes show no more 
stars than those of moderate size, while the few stars seen are 
projected on an intensely dark background. Sir William Her- 
schel, Humboldt, Sir John Herschel, R. A. Proctor, and many 
living astronomers hold that, in these dark areas, rifts, and 
patches, we see completely through our stellar universe into the 
starless depths of space beyond. 

(3) Then we have the remarkable fact that the steady in- 
crease in the number of stars down to the ninth or tenth magni- 
tudes following one constant ratio, either gradually or suddenly 
changes, so that the total number from the tenth down to the 
seventeenth magnitude is only about one-tenth of what it would 
have been had the same .ratio of increase continued. The con- 



ARE THE STARS INFINITE IN NUMBER? 155 

elusion to be drawn from this fact clearly is, that these faint 
stars are becoming more and more thinly scattered in space, 
while the dark background on which they are usually seen shows 
that, except in the region of the Milky Way, there are not 
multitudes of still smaller invisible stars beyond them. 

(4) The last indication of a limited stellar universe — the 
estimate of numbers by the light-ratio of each successive magni- 
tude — powerfully supports the three preceding arguments. 

The four distinct classes of evidence now adduced must be 
held to constitute, as nearly as the circumstances permit, a 
satisfactory proof that the stellar universe, of which our solar 
system forms a part, has definite limits ; and that a full knowl- 
edge of its form, structure, and extent, is not beyond the 
possibility of attainment by the astronomers of the future. 



CHAPTER VIII 

(DUE RELATION TO THE MILKY WAY 

We now approach what may be termed the very heart of the 
subject of our inquiry, the determination of how we are actually 
situated within this vast but finite universe, and how that posi- 
tion is likely to affect our globe as being the theatre of the 
development of life up to its highest forms. 

We begin with our relation to the Milky Way, which we have 
fully described in our fourth chapter, because it is by far the 
most important feature in the whole heavens. Sir John Her- 
schel termed it " the ground-plane of the sidereal system " ; and 
the more it is studied the more we become convinced that the 
whole of the stellar universe — stars, clusters of stars, and 
nebulas — are in some way connected with it, and are probably 
dependent on it or controlled by it. Not only does it contain 
a greater number of stars of the higher magnitudes than any 
other part of the heavens of equal extent, but it also comprises 
a great preponderance of star-clusters, and a great extent of 
diffused nebulous matter, besides the innumerable myriads of 
minute stars which produce its characteristic cloud-like appear- 
ance. It is also the region of those strange outbursts forming 
new stars ; while gaseous stars of enormous bulk — some probably 
a thousand or even ten thousand times that of our sun, and of 

156 



OUR RELATION TO THE MILKY WAY 157 

intense heat and brilliancy — are more abundant there than in 
any other part of the heavens. It is now almost certain that 
these enormous stars and the myriads of minute stars just 
visible with the largest telescopes, are actually intermingled, and 
together constitute its essential features ; in which case the 
fainter stars are really small and cannot be far apart, forming, 
as it were, the first aggregations of the nebulous substratum, 
and perhaps supplying the fuel which keeps up the intense 
brilliancy of the giant suns. If this is so, then the Galaxy 
must be the theatre of operation of vast forces, and of con- 
tinuous combinations of matter, which escape our notice owing 
to its enormous distance from us. Among its millions of minute 
telescopic stars, hundreds or thousands may appear or dis- 
appear yearly without being perceived by us, till the photo- 
graphic charts are completed and can be minutely scrutinised at 
short intervals. As undoubted changes have occurred in many 
of the larger nebulas during the last fifty years, we may 
anticipate that analogous changes will soon be noted in the 
stars and the nebulous masses of the Milky Way. Dr. Isaac 
Roberts has even observed changes in nebulae after such a short 
interval as eight years. 

THE MILKY WAY A GREAT CIRCLE 

Notwithstanding all its irregularities, its divisions, and its 
diverging branches, astronomers are generally agreed that the 
Milky Way forms a great circle in the heavens. Sir John Her- 
schel, whose knowledge of its course was unrivalled, stated that 



158 MAN'S PLACE IN THE UNIVERSE 

it " conforms as nearly as the indefiniteness of its boundary will 
allow it to be fixed to that of a great circle " ; and he gives the 
Right Ascension and Declination of the points where it crosses 
the equinoctial, in figures which define those points as being 
exactly opposite each other. He also defines its northern and 
southern poles by other figures, so as to show that they are the 
poles of a great circle. And after referring to Struve's view 
that it was not a great circle, he says, " I retain my own 
opinion." Professor Newcomb says that its position " is nearly 
always near a great circle of the sphere " ; and again he says : 
" that we are in the galactic plane itself seems to be shown in 
two ways: (1) the equality in the counts of stars on the two 
sides of this plane all the way to its poles; and (2) the fact 
that the central line of the Galaxy is a great circle, which it 
would not be if we viewed it from one side of its central plane " 
(The Stars, p. 317). Miss Clerke, in her History of Astron- 
omy, speaks of " our situation m the galactic plane " as one of 
the undisputed facts of astronomy ; while Sir Norman Lockyer, 
in a lecture delivered in 1899, said, " the middle line of the 
Milky Way is really not distinguishable from a great circle," 
and again in the same lecture, — " but the recent work, chiefly 
of Gould in Argentina, has shown that it practically is a great 
circle." * 

About this fact, then, there can be no dispute. A great 

circle is a circle dividing the celestial sphere into two equal 

portions, as seen from the earth, and therefore the plane of this 

circle must pass through the earth. Of course the whole thing 

1 Nature, October 26, 1899. 



OUR RELATION TO THE MILKY WAY 159 

is on such a vast scale, the Milky Way varying from ten to 
thirty degrees wide, that the plane of its circular course cannot 
be determined with minute accuracy. But this is of little im- 
portance. When carefully laid down on a chart, as in that of 
Mr. Sidney Waters (see end of volume), we can see that its 
central line does follow a very even circular course, conforming 
" as nearly as may be " to a great circle. We are therefore 
certainly well within the space that would be enclosed if its 
northern and southern margins were connected together across 
the vast intervening abyss, and in all probability not far from 
the central plane of that enclosed space. 

THE FORM OF THE MILKY WAY AND OUR POSITION 
ON ITS PLANE 

Although the Galaxy forms a great circle in the heavens from 
our point of view, it by no means follows that it is circular in 
plan. Being unequal in width and irregular in outline, it might 
be elliptic or even angular in shape without being at all obvi- 
ously so to us. If we were standing in an open plain or field 
two or three miles in diameter, and bounded in every direction 
by woods of very irregular height and density and great di- 
versity of tint, we should find it difficult to judge of the shape 
of the field, which might be either a true circle, an oval, a 
hexagon, or quite irregular in outline, without our being able 
to detect the exact shape unless some parts were very much 
nearer to us than others. Again, just as the woods bounding 
the field might be either a narrow belt of nearly uniform width, 



160 MAN'S PLACE IN THE UNIVERSE 

or might in some places be only a few yards wide and in others 
stretch out for miles, so there have been many opinions as to 
the width of the Milky Way in the direction of its plane, that 
is, in the direction in which we look towards it. Lately, how- 
ever, as the result of long-continued observation and study, 
astronomers are fairly well agreed as to its general form and 
extent, as will be seen by the following statements of fact and 
reasoning. 

Miss Clerke, after giving the various views of many as- 
tronomers — and as the historian of modern astronomy her 
opinion has much weight — considers that the most probable 
view of it is, that it is really very much what it seems to us — 
an immense ring with streaming appendages extending from the 
main body in all directions, producing the very complex effect 
we see. The belief seems to be now spreading that the whole 
universe of stars is spherical or spheroidal, the Milky Way 
being its equator, and therefore in all probability circular or 
nearly so in plan ; and it is also held that it must be rotating — 
perhaps very slowly — as nothing else can be supposed to have 
led to the formation of such a vast ring, or can preserve it when 
formed. 

Professor Newcomb considers, from the numbers of the stars 
in all directions towards the Milky Way being approximately 
equal, that there cannot be much difference in our distance from 
it in various directions. It would follow that its plan is ap- 
proximately circular or broadly elliptic. The existence of 
ring-nebula? may be held to render such a form probable. 

Sir Norman Lockyer gives facts which tend in the same 



OUR RELATION TO THE MILKY WAY 161 

direction. In an article in Nature of November 8, 1900, he 
says : " We find that the gaseous stars are not only confined to 
the Milky Way, but they are the most remote in every direction, 
in every galactic longitude; all of them have the smallest 
proper motion." And again, referring to the hottest stars 
being equally remote on all sides of us, he says : " It is because 
we are in the centre, because the solar system is in the centre, 
that the observed effect arises." He also considers that the 
ring-nebula in Lyra nearly represents the form of our whole 
system ; and he adds : " We practically know that in our system 
the centre is the region of least disturbance, and therefore cooler 
conditions." 

These various facts and conclusions of some of the most 
eminent astronomers all point to one definite inference, that our 
position, or that of the solar system, is not very far from the 
centre of the vast ring of stars constituting the Milky Way, 
while the same facts imply a nearly circular form to this ring. 
Here, more than as regards our position in the plane of the 
Galaxy, there is no possibility of precise determination; but it 
is quite certain that if we were situated very far away from the 
centre, say, for instance, one-fourth of its diameter from one 
side of it and three-fourths from the other, the appearances 
would not be what they are, and we should easily detect the 
eccentricity of our position. Even if we were one-third the 
diameter from one side and two-thirds from the other, it will, I 
think, be admitted that this also would have been ascertained 
by the various methods of research now available. We must, 
therefore, be somewhere between the actual centre and a circle 



162 MAN'S PLACE IN THE UNIVERSE 

whose radius is one-third of the distance to the Milky Way. 
But if we are about midway between these two positions, we shall 
only be one-sixth of the radius or one- twelfth of the diameter of 
the Milky Way from its exact centre ; and if we form part of a 
cluster or group of stars slowly revolving around that centre, 
we should probably obtain all the advantages, if any, that may 
arise from a nearly central position in the entire star-system. 

This question of our situation within the great circle of 
the Milky Way is of considerable importance from the point of 
view I am here suggesting, so that every fact bearing upon it 
should be noted; and there is one which has not, I think, been 
given the full weight due to it. It is generally admitted that 
the greater brilliancy of some parts of the Milky Way is no 
indication of nearness, because surfaces possess equal brilliancy 
from whatever distance they are seen. Thus each planet has 
its special brilliancy or reflective power, technically termed its 
" albedo," and this remains the same at all distances if the other 
conditions are similar. But notwithstanding this well-known 
fact, Sir John Herschel's remark that the greater brightness of 
the southern Milky Way " conveys strongly the impression of 
greater proximity," and therefore, that we are excentrically 
placed in its plane, has been adopted by many writers as if it 
were the statement of a fact, or at least a clearly expressed 
opinion, instead of being a mere " impression," and really a 
misleading one. I therefore wish to adduce a phenomenon which 
has a real bearing on the question. It is evident that, if the 
Milky Way were actually of uniform width throughout, then 
differences of apparent width would indicate differences of dis- 



OUR RELATION TO THE MILKY WAY 163 
tance. In the parts nearer to us it would appear wider, where 
more remote, narrower; but in these opposite directions there 
would not necessarily be any differences in brightness. We 
should, however, expect that in the parts nearer to us the lucid 
stars, as well as those within any definite limits of magnitude, 
would be either more numerous or more wide apart on the aver- 
age. No such difference as this, however, has been recorded ; but 
there is a peculiar correspondence in the opposite portions of the 
Galaxy which is very suggestive. In the beautiful charts of the 
Nebulae and Star Clusters by the late Mr. Sidney Waters, pub- 
lished by the Royal Astronomical Society and here reproduced 
by their permission (see end of volume), the Milky Way is de- 
lineated in its whole extent with great detail and from the best 
authorities. These charts show us that, in both hemispheres, 
it reaches its maximum extension on the right and left margins 
of the charts, where it is almost equal in extent; while in the 
centre of each chart, that is, at its nearest points to the north and 
south poles respectively, it is at its narrowest portion ; and, 
although this part in the southern hemisphere is brightest and 
most strongly defined, yet the actual extent, including the fainter 
portions, is, again, not very unequal in the opposite segments. 
Here we have a remarkable and significant symmetry in the pro- 
portions of the Milky Way, which, taken in connection with the 
nearly symmetrical scattering of the stars in all parts of the vast 
ring, is strongly suggestive of a nearly circular form and of our 
nearly central position within its plane. There is one other 
feature in this delineation of the Milky Way which is worthy 
of notice. It has been the universal practice to speak of it as 



164 MAN'S PLACE IN THE UNIVERSE 

being double through a considerable portion of its extent, and 
all the usual star-maps show the division greatly exaggerated, 
especially in the northern hemisphere ; and this division was con- 
sidered so important as to lead to the cloven-disc theory of its 
form, or that it consisted of two separate irregular rings, the 
nearer one partly hiding the more distant; while various spiral 
combinations were held by others to be the best way of explaining 
its complex appearance. But this newer map, reduced from a 
large one by Lord Rosse's astronomer, Dr. Boeddicker, who de- 
voted five years to its delineation, shows us that there is no actual 
division in any portion of it in the northern hemisphere, but that 
everywhere, throughout its whole width, it consists of numerous 
intermingled streams and branches, varying greatly in lumi- 
nosity, and with many faint or barely distinguishable extensions 
along its margins, yet forming one unmistakable nebulous belt; 
and the same general character applies to it in the southern 
hemisphere as delineated by Dr. Gould. 

Another feature, which is well shown to the eye by these more 
accurate maps, is the regular curvature of the central line of the 
Milky Way. We can judge of this almost sufficiently by the 
eye ; but if, with a pair of compasses, we find the proper radius 
and centre of curvature, we shall see that the true circular curve 
is always in the very centre of the nebulous mass, and the same 
radius applied in the same manner to the opposite hemisphere 
gives a similar result. It will be noted that as the Milky Way 
is obliquely situated on these charts, the centre of the curve will 
be about in R. A. Oh. 40m. in the map of the southern hemisphere, 
and in R. A. 12h. 40m. in that of the northern hemisphere; 



OUR RELATION TO THE MILKY WAY 165 

while the radius of curvature will be about the length of the 
chord of eight hours of R. A. as measured on the margin of the 
maps. This great regularity of curve of the central line of the 
Galaxy strongly suggests rotation as the only means by which it 
could have originated and be maintained. 

THE SOLAR CLUSTER 

Astronomers are now generally agreed that there is a cluster 
of stars of which our sun forms a part, though its exact dimen- 
sions, form, and limits are still under discussion. Sir William 
Herschel long ago arrived at the conclusion that the Milky Way 
" consists of stars very differently scattered from those imme- 
diately around us." Dr. Gould believed that there were about 
five hundred bright stars much nearer to us than the Milky Way, 
which he termed the solar cluster. And Miss Clerke observes 
that the actual existence of such a cluster is indicated by the fact 
that " an enumeration of the stars in photometric order discloses 
a systematic excess of stars brighter than the 4th magnitude, 
making it certain that there is an actual condensation in the 
neighbourhood of the sun — that the average allowance of cubical 
space per star is smaller within a sphere enclosing him with a 
radius, say, of 140 light-years, than further away." * 

But the most interesting inquiry into this subject is that by 
Professor Kapteyn of Groningen, one of the most painstaking stu- 
dents of the distribution of the stars. He founds his conclusions 
mainly on the proper motions of the stars, this being the best 
1 The System of the Stars, p. 385. 



166 MAN'S PLACE IN THE UNIVERSE 

general indication of distance in the absence of actual determina- 
tion of parallax. He made use of the proper motions and the 
spectra of more than two thousand stars, and he finds that a con- 
siderable body of stars having large proper motions, and also 
presenting the solar type of spectra, surround our sun in all 
directions, and show no increased density, as the more distant 
stars do, towards the Milky Way. He finds also that towards the 
centre of this cluster stars are far closer together than near its 
outer limits (he says there are ninety-eight times as many), that 
it is roughly spherical in shape, and that the maximum compres- 
sion is, as nearly as can be ascertained, at the centre of the circle 
of the Milky Way, while the sun is at some distance away from 
this central point. 1 

It is a very suggestive fact that most of the stars belonging 
to this cluster have spectra of the solar type, which indicates that 
they are of the same general chemical constitution as our sun, 
and are also at about the same stage of evolution ; and this may 
well have arisen from their origin in a great nebulous mass sit- 
uated at or near the centre of the galactic plane, and probably 
revolving round their common centre of gravity. 

As Kapteyn's result was based on materials which were not so 
full or reliable as those now available, Professor S. Newcomb has 
examined the question himself, using two recent lists of stars, one 
limited to those having proper motions of 10" a year, of which 
there are 295, and the other of nearly 1500 stars with " appre- 
ciable proper motions." They are situated in two zones, each 

x This account of Professor Kapteyn's research is taken from an article by 
Miss A. M. Clerke in Knowledge, April, 1893. 



OUR RELATION TO THE MILKY WAY 167 

about 5° in breath and cutting across the Milky Way in different 
parts of its course. They afford, therefore, a good test of the 
distribution of these nearer stars with regard to the Galaxy. 
The result is, that on the average these stars are not more numer- 
ous in or near the Milky Way than elsewhere; and Professor 
Newcomb expresses himself on this point as follows : " The con- 
clusion is interesting and important. If we should blot out from 
the sky all the stars having no proper motion large enough to 
be detected, we should find remaining stars of all magnitudes; 
but they would be scattered almost uniformly over the sky, and 
show little or no tendency to crowd towards the Galaxy, unless, 
perhaps, in the region near 19h. of Right Ascension." 1 

A little consideration will show that, as the stars of all mag- 
nitudes which are, on the average, nearest to us are spread over 
the sky in " all directions " and " almost uniformly," this neces- 
sarily implies that they form a cluster or group, and that our 
sun is somewhere not very far from the centre of this group. 
Again, Professor Newcomb refers to " the remarkable equality 

x The Stars, p. 2o6. The region here referred to is that where the Milky- 
Way has its greatest width (though nearly as wide in the part exactly oppo- 
site), and where it may perhaps extend somewhat in our direction. 

Miss A. M. Clerke informs me that in April, 1901, Kapteyn withdrew the 
conclusions arrived at in 1893, as being founded on illegitimate reasoning as 
to the relation of parallaxes to proper motions. But as this relation is still 
accepted, under certain limitations, by Professor Newcomb and other astron- 
omers, who have arrived independently at very similar results, it seems not 
improbable that, after all, Professor Kapteyn's conclusions may not require 
very much modification. Professor Newcomb also tells us {The Stars, p. 
214, footnote) that he has seen the latest of Professor Kapteyn's papers, 
down to 1901; but he does not therefore express any doubt as to his own 
conclusions as here referred to. 



168 MAN'S PLACE IN THE UNIVERSE 

in the number of stars in opposite directions from us. We do 
not detect any marked difference between the numbers lying 
round the opposite poles of the Galaxy, nor, so far as known, be- 
tween the star-density in different regions at equal distances from 
the Milky Way" (The Stars, p. 315). And again he refers 
to the same question at p. 317, where he says : " So far as we can 
judge from the enumeration of the stars in all directions, and 
from the aspect of the Milky Way, our system is near the centre 
of the stellar universe." 

It will, I think, now be clear to my readers that the four main 
astronomical propositions stated in my article which appeared 
in the New York Independent and in the Fortnightly Review, 
and which were either denied or declared to be unproved by my 
astronomical critics, have been shown to be supported by so many 
converging lines of evidence, that it is no longer possible to deny 
that they are, at least provisionally, fairly well established. 
These facts are, (1) that the stellar universe is not of infinite 
extent ; ( 2 ) that our sun is situated in the central plane of the 
Milky Way; (3) that it is also situated near to the centre of 
that plane; (4) that we are surrounded by a group or cluster 
of stars of unknown extent, which occupy a place not far removed 
from the centre of the galactic plane, and therefore, near to the 
centre of our universe of stars. 

Not only are these four propositions each supported by con- 
verging lines of evidence, including some which I believe have 
not before been adduced in their support, but a number of astron- 
omers, admittedly of the first rank, have arrived at the same 
conclusions as to the bearing of the evidence, and have expressed 



OUR RELATION TO THE MILKY WAY 169 

their convictions in the clearest manner, as quoted by me. It is 
their conclusions which I appeal to and adopt ; yet my two chief 
astronomical critics positively deny that there is any valid evi- 
dence of the finiteness of the stellar universe, which one of them 
terms " a myth," and he even accuses me of having started it. 
Both of them, however, agree in stating very strongly one objec- 
tion to my main thesis — that our central position (not neces- 
sarily at the precise centre) in the stellar universe has a meaning 
and a purpose, in connection with the development of life and of 
man upon this earth, and, so far as we know, here only. With 
this one objection, the only one that in my opinion has the slight- 
est weight, I will now proceed to deal. 

THE SUN'S MOTION THROUGH SPACE 

The two astronomers who did me the honour to criticise my origi- 
nal article, laid the greatest stress on the fact, that even if I had 
proved that the sun now occupied a nearly central position in 
the great star-system, it was really of no importance whatever, 
because, at the rate the sun was travelling, " five million years ago 
we were deep in the actual stream of the Milky Way ; five million 
years hence we shall have completely crossed the gulf which it 
encircles, and again be a member of one of its constituent groups, 
but on the opposite side. And ten million years are regarded by 
geologists and biologists as but a trifle on account to meet their 
demands upon the bank of Time." Thus speaks one of my 
critics. The other is equally crushing. He says : " If there 
is a centre to the visible universe, and if we occupy it to-day, we 



170 MAN'S PLACE IN THE UNIVERSE 

certainly did not do so yesterday, and shall not do so to-morrow. 
The Solar System is known to be moving among the stars with a 
velocity which would carry us to Sirius within 100,000 years, 
if we happened to be travelling in his direction, as we are not. 
In the 50 or 100 million years during which, according to geol- 
ogists, this earth has been a habitable globe, we must have passed 
by thousands of stars on the right hand and on the left. . . . 
In his eagerness to limit the universe in space, Dr. Wallace kas 
surely forgotten that it is equally important, for his purpose, 
to limit it in time; but incomparably more difficult in the face 
of ascertained facts. ... Indeed, so far from our having 
tranquilly enjoyed a central position in unbroken continuity 
for scores or perhaps hundreds of millions of years, we should 
in that time have traversed the universe from boundary to 
boundary." 1 

Now the average reader of these two criticisms, taking account 
of the high official position of both writers, would accept their 
statements of the case as being demonstrated facts, requiring 
no qualification whatever, and would conclude that my whole 
argument had been thereby rendered worthless, and all that I 
founded upon it a fantastic dream. But if, on the other hand, 
I can show that their stated facts as to the sun's motion are by 
no means demonstrated, because founded upon assumptions which 
may be quite erroneous; and further, that if the facts should 
turn out to be substantially correct, they have both omitted to 
state well-known and admitted qualifications which render the 
conclusions they derive from the facts very doubtful, then the 
1 See Knowledge and The Fortnightly Review of April, 1903. 



OUR RELATION TO THE MILKY WAY 171 

average reader will learn the valuable lesson that official advo- 
cacy, whether in medicine, law, or science, is never to be accepted 
till the other side of the case has been heard. Let us see, there- 
fore, what the facts really are. 

Professor Simon Newcomb calculates that, if there are one 
hundred million stars in the stellar universe each five times the 
mass of our sun, and spread over a space which light would re- 
quire thirty thousand years to cross, then any mass traversing 
such a system with a velocity of more than twenty-five miles a 
second would fly off into infinite space never to return. Now 
as there are many stars which have, apparently, very much more 
than this velocity, it would follow that the visible universe is un- 
stable. It also implies that these great velocities were not ac- 
quired in the system itself, but that the bodies which possess them 
must have entered it from without, thus requiring other universes 
as the feeders of our universe. 

For the accuracy of the above statement the authority of Pro- 
fessor Newcomb is an ample guarantee ; but there may be modi- 
fications required in the data on which it is founded, and these 
may greatly alter the result. If I do not mistake, the estimate 
of a hundred million stars is founded on actual counts or esti- 
mates of stars of successive magnitudes in different parts of 
the heavens, and it does not include either those of the denser 
star-clusters nor the countless millions just beyond the reach 
of telescopes in the Milky Way. Neither does it make allow- 
ance for the dark stars supposed by some astronomers to be 
many times more numerous than the bright ones, nor for the 
vast number of the nebulae, great and small, in calculating the 



172 MAN'S PLACE IN THE UNIVERSE 

total mass of the stellar system. 1 In his latest work Professor 
Newcomb says, " The total number of stars is to be counted by 
hundreds of millions " ; and hence the controlling power of the 
system on bodies within it will be many times greater than that 
given above, and might even be ample to retain within its bounds 
such a rapidly moving star as Arcturus, which is believed to be 
travelling at the rate of more than three hundred miles a second. 
But there is another very important limitation to the conclusions 
to be drawn from Professor Newcomb's calculation. It assumes 
the stars to be nearly uniformly distributed through the whole 
of the space to which the system extends. But the facts are very 
different. The existence of clusters, some of which comprise 
many thousands of stars, is one example of irregularity of distri- 
bution, and any one of these larger clusters would probably be 
able to change the course of even the swiftest stars passing near 
it. The larger nebulae might have the same effect, since the late 
Mr. Ranyard, taking all his data so as to produce a minimum 
result, calculated the probable mass of the Orion nebula to be 
four and a half million times that of the sun, and there may be 
many other nebulae equally large. But far more important is 
the fact of the vast ring of the Milky Way, which is now uni- 
versally held by astronomers to be, not only apparently but 
really, more densely crowded with stars and also with vast masses 
of nebulous matter than any other part of the heavens, so that it 

1 Sir R. Ball in an article in Good Words (April, 1903) says that lumin- 
osity is an exceptional phenomenon in nature, and that luminous stars are 
but the glow-worms and fire-flies of the universe, as compared with the 
myriads of other animals. 



OUR RELATION TO THE MILKY WAY 173 

may possibly comprise within itself a very large proportion of 
the whole of the matter of the visible universe. This is rendered 
more probable by the fact that the great majority of star-clus- 
ters lie along its course, most of the huge gaseous stars belong 
to it, while the occurrence there only of " new stars " is evidence 
of a superabundance of matter in various forms leading to fre- 
quent heat-producing collisions, just as the frequent occurrence 
of meteoric showers on our earth is evidence of the superabund- 
ance of meteoric matter in the solar system. 

It is recognised by mathematicians that within any great sys- 
tem of bodies subject to the law of gravitation there can be no 
such thing as motion of any of them in a straight line ; neither 
can any amount of motion arise within such a system through 
the action of gravitation alone capable of carrying any of its 
masses out of the system. The ultimate tendency must be 
towards condensation rather than towards dispersal. 

It seems, therefore, only reasonable to consider whatever mo- 
tions and whatever velocities we find among the stars, as having 
been produced by the gravitative power of the larger aggrega- 
tions, modified perhaps by electrical repulsive forces, by colli- 
sions, and by the results of those collisions; and we may look 
to the changes now visibly going on in some of the nebulae and 
clusters as indications of the forces that have probably brought 
about the actual condition of the whole stellar universe. 

If we examine the beautiful photographs of nebulae by Dr. 
Roberts and other observers, we find that they are of many forms. 
Some are extremely irregular and almost like patches of cirrus 
clouds, but a large number are either distinctly spiral in form, or 



174 MAN'S PLACE IN THE UNIVERSE 

show indications of becoming spiral, and this has been found to 
be the case even with some of the large irregular nebulae. Then 
again we have numerous ring-formed nebulae, usually with a star 
involved in dense nebulosity in the centre, separated by a dark 
space of various widths from the outer ring. All these kinds 
of nebulae have stars involved in them, and apparently forming 
part of their structure, while others which do not differ in ap- 
pearance from ordinary stars are believed by Dr. Roberts to lie 
between us and the nebula. In the case of many of the spiral 
nebulae, stars are often strung along the coils of the spiral, while 
other curved lines of stars are seen just outside the nebula, so 
that it is impossible to avoid the conclusion that both are really 
connected with it, the outer lines of stars indicating a former 
greater extension of the nebula whose material has been used up 
in the growth of these stars. Some of these spiral nebulae show 
beautifully regular convolutions, and these visually have a large 
central star-like mass, as in M. 100 Comae and I. 84 Comae, in 
Vol. II. PL 14 of Dr. Roberts's photographs. The straight 
white streaks across the nebula of the Pleiades and some others 
are believed by Dr. Roberts to be indications of spiral nebulae 
seen edgewise. In other cases, clusters of stars are more or less 
nebulous, and the arrangement of the stars seems to indicate their 
development from a spiral nebula. It is to be noted that many 
of the objects classed as planetary nebulae by Sir John Herschel 
are shown by the best photographs to be really of the ring-type, 
though often with a very narrow division between the ring and 
the central mass. This form may therefore be of frequent 
occurrence. 



OUR RELATION TO THE MILKY WAY 175 

But if this annular form with some kind of central nucleus, 
often very large, is produced under certain conditions by the 
action of the ordinary laws of motion upon more or less extensive 
masses of discrete matter, why may not the same laws acting 
upon similar matter once dispersed over the whole extent of the 
existing stellar universe, or even beyond what are now its furthest 
limits, have led to the aggregation of the vast annular formation 
of the Milky Way, with all the subordinate centres of concen- 
tration or dispersal to be found within or around it? And if 
this is a reasonable conception, may we not hope that by a con- 
centration of attention upon a few of the best marked and most 
favourably situated annular and spiral systems, sufficient knowl- 
edge of their internal motions may be obtained which may serve 
as a guide to the kind of motion we may expect to find in the 
great galactic ring and its subordinate stars? We may then 
perhaps discover that the proper motions of the stars, and of 
our sun, which now seem so erratic, are really all parts of a 
series of orbital movements limited and controlled by the forces 
of the great system to which they belong, so that, if not mathe- 
matically stable, they may yet be sufficiently so to endure for some 
thousand millions of years. 

It is a suggestive fact that the calculated position of the 
" solar apex " — the point towards which our sun appears to 
move — is now found to be much more nearly in the plane of the 
Milky Way than the position first assigned to it, and Professor 
Newcomb adopts, as most likely to be accurate, a point near the 
bright star Vega in the constellation Lyra. Other calculators 
have placed it still further east, while Rancken and Otto Stumpe 



176 MAN'S PLACE IN THE UNIVERSE 

assign it a position actually in the Milky Way; and Mr. G. C. 
Bompas concludes that the sun's plane of motion nearly coincides 
with that of the Galaxy. M. Rancken found that 106 stars near 
the Milky Way showed, in their very small proper motions, a 
drift along it in a direction from Cassiopeia towards Orion, and 
this, it is supposed, may be partly due to our sun's motion in an 
opposite direction. 

In many other parts of the heavens there are groups of stars 
which have almost identical proper motions — a phenomenon 
which the late R. A Proctor termed " star-drift " ; and he espe- 
cially pointed out that five of the stars of the Great Bear were 
all drifting in the same direction; and although this has been 
denied by later writers, Professor Newcomb, in his recent book 
on The Stars, declares that Proctor was right, and explains that 
the error of his critics was due to not making allowance for the 
divergence of the circles of right ascension. The Pleiades are 
another group, the stars of which drift in the same direction, and 
it is a most suggestive fact that photographs now show this clus- 
ter to be embedded in a vast nebula, which, therefore, has also a 
proper motion ; but some of the smaller stars do not partake of 
it. Three stars in Cassiopeia also move together, and no doubt 
many other similarly connected groups remain to be discovered. 

These facts have a very important bearing on the question of 
the motion of cur sun in space. For this motion has been deter- 
mined by comparing the motions of large numbers of stars which 
are assumed to be wholly independent of each other, and to move, 
as it were, at random. Miss A. M. Clerke, in her System of the 
Stars, puts this point very clearly, as follows : " For the assump- 



OUR RELATION TO THE MILKY WAY 177 

tion that the absolute movements of the stars have no preference 
for one direction over another, forms the basis of all investiga- 
tions hitherto conducted into the translatory advance of the solar 
system. The little fabric of laboriously acquired knowledge 
regarding it at once crumbles if that basis has to be removed. 
In all investigations of the sun's movement, the movements of the 
stars have been regarded as casual irregularities ; should they 
prove to be in any visible degree systematic, the mode of treat- 
ment adopted (and there is no other at present open to us) be- 
comes invalid, and its results null and void. The point is then 
of singular interest, and the evidence bearing upon it deserves 
our utmost attention." 

Mr. W. H. S. Monck, a well-known astronomer, takes the same 
view. He says : " The proof of this motion rests on the assump- 
tion that if we take a sufficient number of stars, their real motions 
in all directions will be equal, and that therefore the apparent 
preponderances which we observe in particular directions result 
from the real motion of the sun. But there is no impossibility in a 
systematic motion of the majority of the stars used in these re- 
searches which might reconcile the observed facts with a motion- 
less sun. And, in the second place, if the sun is not in the exact 
centre of gravity of the universe, we might expect him to be 
moving in an orbit around this centre of gravity, and our ob- 
servations on his actual motion are not sufficiently numerous or 
accurate to enable us to affirm that he is moving in a right line 
rather than such an orbit." 

Now this " systematic motion," which would render all cal- 
culations as to the sun's motion inaccurate or even altogether 



178 MAN'S PLACE IN THE UNIVERSE 

worthless, is by many astronomers held to be an observed reality. 
The star-drift, first pointed out by Proctor, has been shown to 
exist in many other groups of stars, while the curious arrange- 
ments of stars all over the heavens in straight lines, or regular 
curves, or spirals, strongly suggests a wide extension of the same 
kind of relation. But even more extensive systematic movements 
have been observed or suggested by astronomers. Sir D. Gill, 
by an extensive research, believes that he has found indications of 
a rotation of the brighter fixed stars as a whole in regard to the 
fainter fixed stars as a whole. Mr. Maxwell Hall has also found 
indications of a movement of a large group of stars, including 
our sun, around a common centre, situated in a direction towards 
Epsilon Andromedae, and at a distance of about 490 years of 
light-travel. These last two motions are not yet established; 
but they seem to prove two important facts — (a) that eminent 
astronomers believe that some systematic motions must exist 
among the stars, or they would not devote so much labour to the 
search for them; and (b) that extensive systematic motions of 
some kind do exist, or even these results would not have been ob- 
tained. 

Mr. W. W. Campbell, of the Lick Observatory, thus remarks 
on the uncertainty of determinations of the sun's motions : " The 
motion of the solar system is a purely relative quantity. It 
refers to specified groups of stars. The results for various 
groups may differ widely, and all be correct. It would be easy 
to select a group of stars with reference to which the solar motion 
would be reversed 180° from the values assigned above " (Astro- 
physical Journal, vol. xiii. p. 87. 1901). 



OUR RELATION TO THE MILKY WAY 179 

It must be remembered that, within a uniform cluster of stars, 
each moving round the common centre of gravity of the whole 
cluster, Kepler's laws do not prevail, the law being that the angu- 
lar velocities are all identical, so that the more distant stars move 
faster than those nearer the centre, subject to modifications, how- 
ever, due to the varying density of the cluster. But if the clus- 
ter is nearly globular, there must be stars moving round the 
centre in every plane, and this would lead to apparent motions in 
many directions as viewed by us, although those which were mov- 
ing in the same plane as ourselves would, when compared with 
remote stars outside the cluster, appear to be all moving in the 
same direction and at the same rate, forming, in fact, one of those 
drifting systems of stars already referred to. Again, if in the 
process of formation of our cluster, smaller aggregations already 
having a rotatory motion were drawn into it, this might lead to 
their revolving in an opposite direction to those which were 
formed from the original nebula, thus increasing the diversities 
of apparent motion. 

The evidence now briefly set forth fully justifies, I submit, the 
remarks as to the statements of my astronomical critics at the 
beginning of this section. They have both given the accepted 
views as to direction and rate of movement of our sun without 
any qualification whatever, as if they were astronomical facts of 
the same certainty and the same degree of accuracy as the sun's 
distance from the earth; and they will assuredly have been so 
understood by the great body of non-mathematical readers. It 
appears, however, if the authorities I have quoted are right, that 
the whole calculation rests upon certain assumptions, which are 



180 MAN'S PLACE IN THE UNIVERSE 

certainly to some extent, and may be to a very large extent, 
erroneous. This is my reply to one part of their criticism. 

In the next place, they both assert, or imply, not only that 
the sun's motion is now in a straight line, but that it has been 
in a straight line from some enormously remote period when it 
first entered the stellar system on one side, and will so continue 
to move till it reaches the utmost bounds of that system on the 
other side. And this is stated by them both, not as a possi- 
bility, but as a certainty. They use such terms as " must " and 
" will be," leaving no room for any doubt whatever. But such a 
result implies the abrogation of the law of gravitation, since 
under its action motion in a straight line in the midst of thou- 
sands or millions of suns of various sizes is an absolute impossi- 
bility; while it also implies that the sun must have been started 
on its course from some other system outside the Milky Way, 
with such a precise determination of direction as not to collide 
with, or even make a near approach to, any one of the suns or 
clusters of suns, or vast nebulous masses, during its passage 
through the very midst of the stellar universe. 

This is my reply to the main point of their criticism, and I 
think I am justified in saying that nothing in my whole article is 
so demonstrably baseless as the statements I have now examined. 

Considering then the whole bearing of the evidence, I refuse 
to accept the unsupported dicta of those who would have us be- 
lieve that our admitted position not far from the centre of the 
stellar universe is a mere temporary coincidence of no signifi- 
cance whatever : or that our sun and hosts of other similar orbs 



OUR RELATION TO THE MILKY WAY 181 

near to us have come together by an accident, and are being dis- 
persed into surrounding space, never to meet again. Until this 
is proved by indisputable evidence, it seems to me far more prob- 
able that we are moving in an orbit of some kind around the 
centre of gravity of a vast cluster, as determined by the investi- 
gations of Kapteyn, Newcomb, and other astronomers ; and, con- 
sequently, that the nearly central position we now occupy may 
be a permanent one. For even if our sun's orbit should have 
a diameter a thousand times that of Neptune, it would be but 
a small fraction of the diameter of the Milky Way ; while so vast 
is the scale of our universe, that it might be even a hundred thou- 
sand times as great and still leave us deeply immersed in the solar 
cluster, and very much nearer to the dense central portion than 
to its more diffused outer regions. 

Here the subject may be left for the present. After having 
studied the evidence afforded by the essential conditions of life- 
development on the earth, and the numerous indications that 
these conditions do not exist on any of the other planets of the 
solar system, it may be again touched upon in a general review of 
the conclusions arrived at. 



CHAPTER IX 

THE UNIFORMITY OF MATTER AND ITS LAWS THROUGHOUT THE 
STELLAR UNIVERSE 

I have shown in the second chapter of this work that none of 
the previous writers on the question of the habitability of the 
other planets have really dealt with the subject in any adequate 
manner, since not only do they appear to be quite unaware of 
the delicate balance of conditions which alone renders organic 
life possible on any planet, but they have altogether omitted any 
reference to the fact that not only must the conditions be such 
as to render life possible now, but these conditions must have per- 
sisted during the long geological epochs needed for the slow 
development of life from its most rudimentary forms. It will 
therefore be necessary to enter into some details both as to the 
physical and chemical essentials for a continuous development 
of organic life, and also into the combination of mechanical and 
physical conditions which are required on any planet to render 
such life possible. 

THE UNIFORMITY OF MATTER 

One of the most important and far-reaching of the discoveries 
due to the spectroscope, is that of the wonderful identity of the 
elements and material compounds in earth and sun, stars and 

182 



THE UNIFORMITY OF MATTER, ETC. 183 

nebulae, and also of the identity of the physical and chemical 
laws that determine the states and forms assumed by matter. 
More than half the total number of the known elements have been 
already detected in the sun, including all those which compose 
the bulk of the earth's solid material, with the one exception of 
oxygen. This is a very large proportion when we consider the 
very peculiar conditions which enable us to detect them. For 
we can only recognise an element in the sun when it exists at its 
surface in an incandescent state, and also above its surface in the 
form of a somewhat cooler gas. Many of the elements may 
rarely or never be brought to the surface of so vast a body, or if 
they do sometimes appear there, it may not be in sufficient quan- 
tity or in sufficient purity to produce any bands in the spectro- 
scope, while the cooler gas or vapour may either not be present, 
or be so dispersed as not to produce sufficient absorption to render 
its spectral lines visible. Again, it is believed that many elements 
are dissociated by the intense heat of the sun, and may not be 
recognisable by us, or they may only exist at its surface in a com- 
pound form unknown on the earth ; and in some such way those 
lines of the solar spectrum which remain still unrecognised may 
have been produced. One of these unknown lines was that of 
Helium, a gas found soon afterwards in the rare mineral 
" Clevite " and since detected frequently in many stars. Some 
of the stars have spectra very closely resembling that of the sun. 
The dark lines are almost as numerous, and most of them corre- 
spond accurately with solar lines, so that we cannot doubt their 
having almost exactly the same chemical constitution, and being 
also in the same condition as regards heat and stage of develop- 



184 MAN'S PLACE IN THE UNIVERSE 

ment. Other stars, as we have already stated, exhibit mainly 
lines of hydrogen, sometimes combined with fine metallic lines. 
Of the spectra of the nebula? comparatively little is known, but 
many are decidedly gaseous, while others show a continuous spec- 
trum indicating a more complex constitution. 

But we also obtain considerable knowledge of the matter of 
non-terrestrial bodies by the analysis of the numerous meteorites 
which fall upon the earth. Most of these belong to some of the 
many meteoric streams which circulate round the sun, and which 
may be supposed to give us samples of planetary matter. But 
as it is now believed that many of them are produced by the 
debris of comets, and the orbits of some of these indicate that 
they have come from stellar space and have been drawn into our 
system by the attractive power of the larger planets, it is almost 
certain that the meteoric stones not infrequently bring us matter 
from the remoter regions of space, and probably afford us 
samples of the solid constituents of nebulae, or the cooler stars. 
It is, therefore, a most suggestive fact that none of these meteor- 
ites have been found to contain a single non-terrestrial element, 
although no less than twenty-four elements have been found in 
them ; and it will be of interest to give the list of these, as follows : 
Oxygen, Hydrogen, Chlorine, Sulphur, Phosphorus, Carbon, 
Silicon, Iron, Nickel, Cobalt, Magnesium, Chromium, Manga- 
nese, Copper, Tin, Antimony, Aluminium, Calcium, Potassium, 
Sodium, Lithium, Titanium, Arsenic, and Vanadium. Seven of 
the above, printed in italics, have not yet been found in the sun, 
such as Oxygen, Chlorine, Sulphur, and Phosphorus, which form 
the constituents of many widespread minerals, and they supply 



THE UNIFORMITY OF MATTER, ETC. 185 

important gaps in the series of solar and stellar elements. It 
may be noted that although meteorites have supplied no new ele- 
ments they have furnished examples of some new combinations of 
these elements forming minerals distinct from any found in our 
rocks. 

The fact of the occurrence in meteorites not only of minerals 
which are peculiar to them or are found on the earth, but also 
of structures resembling our breccias, veins, and even slicken- 
side surfaces, has been held to be opposed to the meteoritic theory 
of the origin of suns and planets, because they are, it is said, thus 
proved to be the fragments of suns or worlds, not their primary 
constituents. But these cases are exceptional, and Mr. Sorby, 
who made a special study of meteorites, concluded that their 
materials have usually been in a state of fusion or even of vapour, 
as they now exist in the sun, and that they became condensed into 
minute globular particles, which afterwards collected into larger 
masses, and may have been broken up by mutual impact, and 
again and again become aggregated together — thus presenting 
features which are completely in accordance with the meteoritic 
theory. 

But, quite recently, Mr. T. C. Chamberlin has applied the 
theory of tidal distortion to showing how solid bodies in space, 
without ever coming into actual contact, must sometimes be torn 
apart or disrupted into numerous fragments by passing near 
to each other. Especially when a small body passes near a much 
larger one, there is a certain distance of approach (termed the 
Roche limit) when the rapidly increasing force of gravity will 
be sufficient to tear asunder the smaller body and cause the frag- 



186 MAN'S PLACE IN THE UNIVERSE 

ments either to circulate around it or to be dispersed in space. 1 
In this way, therefore, those larger meteorites which exhibit 
planetary structure may have been produced. Of course they 
would rarely have been true planets attached to a sun, but more 
frequently some of the smaller dark suns, which may possess 
many of the physical characteristics of planets, and of which 
there may be myriads in the stellar spaces. 

On the whole, then, we have positive knowledge of the existence, 
in the sun, stars, and planetary and stellar spaces, of such a large 
proportion of the elements of our globe, and so few indications 
of any not forming part of it, that we are justified in the state- 
ment, that the whole stellar universe is, broadly speaking, con- 
structed of the same series of elementary substances as those we 
can study upon our earth, and of which the whole realm of 
nature, animal, vegetable, and mineral, is composed. The evi- 
dence of this identity of substance is really far more complete 
than we could expect, considering the very limited means of in- 
quiry that we possess ; and we shall, therefore, not be justified in 
assuming that any important difference exists. 

When we pass from the elements of matter to the laws which 
govern it, we also find the clearest proofs of identity. That the 
fundamental law of gravitation extends to the whole physical uni- 
verse is rendered almost certain by the fact that double stars 
move round their common centre of gravity in elliptical orbits 
which correspond well with both observation and calculation. 
That the laws of light are the same both here and in inter-plan- 
etary space is indicated by the fact that the actual measurement 
l The Astrophysical Journal, vol. xiv., July, 1901, p. 17. 



THE UNIFORMITY OF MATTER, ETC. 187 

of the velocity of light on the earth's surface gives a result so 
completely identical with that prevailing to the limits of the 
solar system, that the measurement of the sun's distance, by 
means of the eclipses of Jupiter's satellites combined with the 
measured velocity of light, agrees almost exactly with that ob- 
tained by means of the transits of Venus, or through our nearest 
approach to the planets Mars or Eros. 

Again, the more recondite laws of light are found to be identi- 
cal in sun and stars with those observed within the narrow bounds 
of laboratory experiments. The minute change of position of 
spectral lines caused by the source of light moving towards or 
away from us enables us to determine this kind of motion in the 
most distant stars, in the planets, or in the moon, and these 
results can be tested by the motion of the earth either in its orbit 
or in its rotation ; and these latter tests agree with the theoretical 
determination of what must occur, dependent on the wave-lengths 
of the different dark lines of the solar spectrum determined by 
measurements in the laboratory. 

In like manner, minute changes in the widening or narrowing 
of spectral lines, their splitting up, their increase or decrease in 
number, and their arrangement so as to form flu tings, can all 
be interpreted by experiments in the laboratory, showing that 
such phenomena are due to alterations of temperature, of pres- 
sure, or of the magnetic field, thus proving that the very same 
physical and chemical laws act in the same way here and in the 
remotest depths of space. 

These various discoveries give us the certain conviction that 
the whole material universe is essentially one, both as regards the 



188 MAN'S PLACE IN THE UNIVERSE 

action of physical and chemical laws, and also in its mechanical 
relations of form and structure. It consists throughout of the 
very same elements with which we are so familiar on our earth; 
the same ether whose vibrations bring us light and heat, elec- 
tricity and magnetism, and a whole host of other mysterious 
and as yet imperfectly known forces ; gravitation acts through- 
out its vast extent; and in whatever direction and by whatever 
means we obtain a knowledge of the stellar universe, we find the 
same mechanical, physical, and chemical laws prevailing as upon 
our earth, so that we have in some cases been actually enabled to 
reproduce in our laboratories phenomena with which we had first 
become acquainted in the sun or among the stars. 

We may therefore feel it to be an almost certain conclusion 
that — the elements being the same, the laws which act upon, and 
combine, and modify those elements being the same — organised 
living beings wherever they may exist in this universe must be, 
fundamentally, and in essential nature, the same also. The out- 
ward forms of life, if they exist elsewhere, may vary almost in- 
finitely, as they do vary on the earth; but, throughout all this 
variety of form — from fungus or moss to rose-bush, palm, or oak ; 
from mollusc, worm, or butterfly to humming-bird, elephant, or 
man — the biologist recognises a fundamental unity of substance 
and of structure, dependent on the absolute requirements of the 
growing, moving, developing, living organism, built up of the 
same elements, combined in the same proportions, and subject to 
the same laws. We do not say that organic life could not exist 
under altogether diverse conditions from those which we know or 
can conceive, conditions which may prevail in other universes 



THE UNIFORMITY OF MATTER, ETC. 189 

constructed quite differently from ours, where other substances 
replace the matter and ether of our universe, and where other 
laws prevail. But, within the universe we know, there is not the 
slightest reason to suppose organic life to be possible, except 
under the same general conditions and laws which prevail here. 
We will, therefore, now proceed to describe, very generally, what 
are the conditions essential to the existence and the continuous 
development of vegetable and animal life. 



CHAPTER X 

THE ESSENTIAL CHARACTERS OF THE LIVING ORGANISM 

Before trying to comprehend the physical conditions on any 
planet which are essential for the development and maintenance 
of a varied and complex system of organic life comparable to that 
of our earth, we must obtain some knowledge of what life is, and 
of the fundamental nature and properties of the living organism. 
Physiologists and philosophers have made many attempts to 
define " life," but in most cases in aiming at absolute generality 
they have been vague and uninstructive. Thus De Blainville 
defined it as " The twofold internal movement of composition 
and decomposition, at once general and continuous " ; while Her- 
bert Spencer's latest definition was " Life is the continuous ad- 
justment of internal relations to external relations." But 
neither of these is sufficiently precise, explanatory, or distinc- 
tive, and they might almost be applied to the changes occurring 
in a sun or planet, or to the elevation and gradual formation of 
a continent. One of the oldest definitions, that of Aristotle, 
seems to come nearer the mark : " Life is the assemblage of the 
operations of nutrition, growth, and destruction." But these 
definitions of " life " are unsatisfactory, because they apply to 
an abstract idea rather than to the actual living organism. The 
marvel and mystery of life, as we know it, resides in the 

190 



CHARACTERS OF LIVING ORGANISM 191 

body which manifests it, and this living body the definitions 
ignore. 

The essential points in the living body, as seen in its higher 
developments, are, first, that it consists throughout of highly 
complex but very unstable forms of matter, every particle of 
which is in a continual state of growth or decay ; that it absorbs 
or appropriates dead matter from without ; takes this matter into 
the interior of its body ; acts upon it mechanically and chemically, 
rejecting what is useless or hurtful; and so transforming the 
remainder as to renew every atom of its own structure, internal 
and external, at the same time throwing off, particle by particle, 
all the worn-out or dead portions of its own substance. Secondly, 
in order to be able to do all this, its whole body is permeated 
throughout by branching vessels or porous tissues, by which 
liquids and gases can reach every part and carry on the various 
processes of nutrition and excretion above referred to. As Pro- 
fessor Burdon Sanderson well puts it : " The most distinctive 
peculiarity of living matter as compared with non-living is, that 
it is ever changing while ever the same." And these changes 
are the more remarkable because they are accompanied, and even 
produced, by a very large amount of mechanical work — in ani- 
mals by means of their normal activities in search of food, in 
assimilating that food, in continually renewing and building up 
their whole organism, and in many other ways; in plants by 
building up their structure, which often involves raising tons 
of material high into the air, as in forest trees. As a recent 
writer puts it : " The most prominent, and perhaps the most 
fundamental, phenomenon of life is what may be described as 



192 MAN'S PLACE IN THE UNIVERSE 

the Energy Traffic or the function of trading in energy. The 
chief physical function of living matter seems to consist in 
absorbing energy, storing it in a higher potential state, and 
afterwards partially expending it in the kinetic or active 
form." x 

Thirdly — and perhaps most marvellous of all — all living 
organisms have the power of reproduction or increase, in the 
lowest forms by a process of self -division or " fission," as it is 
termed, in the higher by means of reproductive cells, which, 
though in their earliest stage quite indistinguishable physically 
or chemically in very different species, yet possess the mysterious 
power of developing a perfect organism, identical with its 
parents in all its parts, shapes, and organs, and so wonderfully 
resembling them, that the minutest distinctive details of size, 
form, and colour, in hair or feathers, in teeth or claws, in scales, 
spines, or crests, are reproduced with very close accuracy, though 
often involving metamorphic changes during growth of so 
strange a nature that, if they were not familiar to us but were 
narrated as occurring only in some distant and almost inacces- 
sible region, would be treated as travellers' tales, incredible and 
impossible as those of Sindbad the Sailor. 

In order that the substance of living bodies should be able to 
undergo these constant changes while preserving the same form 
and structure in minute details — that they should be, as it were, 
in a constant state of flux while remaining sensibly unchanged, 
it is necessary that the molecules of which they are built up 
should be so combined as to be easily separated and as easily 
Professor F. J. Allen: What is Life? 



CHARACTERS OF LIVING ORGANISM 193 

united — be, as it is termed, labile or flowing ; and this is brought 
about by their chemical composition, which, while consisting of 
few elements, is yet highly complex in structure, a large num- 
ber of chemical atoms being combined in an endless variety 
of ways. 

The physical basis of life, as Huxley termed it, is protoplasm, 
a substance which consists essentially of only four common ele- 
ments, the three gases, nitrogen, hydrogen, and oxygen, with 
the non-metallic solid, carbon; hence all the special products of 
plants and animals are termed carbon-compounds, and their 
study constitutes one of the most extensive and intricate branches 
of modern chemistry. Their complexity is indicated by the 
fact that the molecule of sugar contains 45, and that of stearine 
no less than 173 constituent atoms. The chemical compounds 
of carbon are far more numerous than those of all the other 
chemical elements combined ; and it is this wonderful variety and 
the complexity of its possible combinations which explain the 
fact, that all the various animal tissues — skin, horn, hair, nails, 
teeth, muscle, nerve, etc., consist of the same four elements (with, 
occasionally, minute quantities of sulphur, phosphorus, lime, 
silica, in some of them), as proved by the marvellous fact that 
these tissues are all produced as well by the grass-eating sheep 
or ox as by the fish- or flesh-eating seal or tiger. And the marvel 
is still further increased when we consider that the innumerable 
diverse substances produced by plants and animals are all formed 
out of the same three or four elements. Such are the endless 
variety of organic acids, from prussic acid to those of the various 
fruits ; the many kinds of sugars, gums, and starches ; the num- 



194 MAN'S PLACE IN THE UNIVERSE 

ber of different kinds of oil, wax, etc. ; the variety of essential 
oils which are mostly forms of turpentines, with such substances 
as camphor, resins, caoutchouc, and gutta-percha ; and the exten- 
sive series of vegetable alkaloids, such as nicotine from tobacco, 
morphine from opium, strychnine, curarine, and other poisons; 
quinine, belladonna, and similar medicinal alkaloids; together 
with the essential principles of our refreshing drinks, tea, coffee, 
and cocoa, and others too numerous to be named here — all alike 
consisting solely of the four common elements from which almost 
our whole organism is built up. If this were not indisputably 
proved, it would scarcely be credited. 

Professor F. J. Allen considers that the most important ele- 
ment in protoplasm, and that which confers upon it its most 
essential properties in the living organism — its extreme mobility 
and transposability — is nitrogen. This element, though inert in 
itself, readily enters into compounds when energy is supplied to 
it, the most striking illustration of which is the formation of 
ammonia, a compound of nitrogen and hydrogen, produced by 
electric discharges through the atmosphere. Ammonia, and cer- 
tain oxides of nitrogen produced in the atmosphere in the same 
way, are the chief sources of the nitrogen assimilated by plants, 
and through them by animals; for although plants are contin- 
ually in contact with the free nitrogen of the atmosphere, they 
are unable to absorb it. By their leaves they absorb oxygen 
and carbon-dioxide to build up their woody tissues, while by their 
roots they absorb water in which ammonia and oxides of nitrogen 
are dissolved, and from these they produce the protoplasm which 
builds up the whole substance of the animal world. The energy 



CHARACTERS OF LIVING ORGANISM 195 

required to produce these nitrogen-compounds is given up by 
them when undergoing further changes, and thus the production 
of ammonia by electricity in the atmosphere, and its being carried 
by rain into the soil, constitute the first steps in that long series 
of operations which culminates in the production of the higher 
forms of life. 

But the remarkable transformations and combinations con- 
tinually going on in every living body, which are, in fact, the 
essential conditions of its lif e, are themselves dependent on certain 
physical conditions which must be always present. Professor 
Allen remarks : " The sensitiveness of nitrogen, its proneness to 
change its state of combination and energy, appear to depend 
on certain conditions of temperature, pressure, etc., which exist 
at the surface of this earth. Most vital phenomena occur be- 
tween the temperature of freezing water and 104° F. If the 
general temperature of the earth's surface rose or fell 72° F. 
(a small amount relatively), the whole course of life would be 
changed, even perchance to extinction." 

Another important, and even more essential fact, in connection 
with life, is the existence in the atmosphere of a small but nearly 
constant proportion of carbonic acid gas, this being the source 
from which the whole of the carbon in the vegetable and animal 
kingdoms is primarily derived. The leaves of plants absorb 
carbonic acid gas from the atmosphere, and the peculiar sub- 
stance, chlorophyll, from which they derive their green colour, 
has the power, under the influence of sunlight, to decompose it, 
using the carbon to build up its own structure and giving out 
the oxygen. In the laboratory the carbon can only be separated 



196 MAN'S PLACE IN THE UNIVERSE 

from the oxygen by the application of heat, under which certain 
metals burn by combining with the oxygen, thus setting free the 
carbon. Chlorophyll has a highly complex chemical structure 
very imperfectly known, but it is said to be only produced when 
there is iron in the soil. 

The leaves of plants, so often looked upon as mere ornamental 
appendages, are among the most marvellous structures in living 
organisms, since in decomposing carbonic acid at ordinary tem- 
peratures they do what no other agency in nature can perform. 
In doing this they utilise a special group of ether-waves which 
alone appear to have this power. The complexity of the proc- 
esses going on in leaves is well indicated in the following quota- 
tion : 

" We have seen how green leaves are supplied with gases, 
water, and dissolved salts, and how they can trap special ether- 
waves. The active energy of these waves is used to transmute 
the simple inorganic compounds into complex organic ones, which 
in the process of respiration are reduced to simpler substances 
again, and the potential energy transformed into kinetic. These 
metabolic changes take place in living cells full of intense activ- 
ities. Currents course through the protoplasm and cell-sap in 
every direction, and between the cells which are also united by 
strands of protoplasm. The gases used and given off in respira- 
tion and assimilation are floated in and out, and each protoplasm 
particle burned or unburned is the centre of an area of disturb- 
ance. Pure protoplasm is influenced equally by all rays: that 
associated with chlorophyll is affected by certain red and violet 
rays in particular. These, especially the red ones, bring about 



CHARACTERS OF LIVING ORGANISM 197 

the dissociation of the elements of the carbonic acid, the assimila- 
tion of the carbon, and the excretion of the oxygen." * 

It is this vigorous life-activity ever at work in the leaves, the 
roots, and the sap-cells, that builds up the plant, in all its won- 
drous beauty of bud and foliage, flower and fruit; and at the 
same time produces, either as useful or waste-products, all that 
wealth of odours and flavours, of colours and textures, of fibres 
and varied woods, of roots and tubers, of gums and oils and 
resins innumerable, that, taken altogether, render the world of 
vegetable life perhaps more varied, more beautiful, more enjoy- 
able, more indispensable to our higher nature than even that of 
animals. But there is really no comparison between them. We 
could have plants without animals; we could not have animals 
without plants. And all this marvel and mystery of vegetable 
life, a mystery which we rarely ponder over because its effects 
are so familiar, is usually held to be sufficiently explained by the 
statement that it is all due to the special properties of proto- 
plasm. Well might Huxley say, that protoplasm is not only a 
substance but a structure or mechanism, a mechanism kept at 
work by solar heat and light, and capable of producing a thou- 
sand times more varied and marvellous results than all the human 
mechanism ever invented. 

But besides absorbing carbonic acid from the atmosphere, 
separating and utilising the carbon and giving out the oxygen, 
plants as well as animals continually absorb oxygen from the 
atmosphere, and this is so universally the case that oxygen is 
said to be the food of protoplasm, without which it cannot con- 
1 Art. "Vegetable Physiology" in Chambers's Encyclopaedia. 



198 MAN'S PLACE IN THE UNIVERSE 

tinue to live ; and it is the peculiar but quite invisible structure of 
the protoplasm which enables it to do this, and also in plants to 
absorb an enormous amount of water as well. 

But although protoplasm is so complex chemically as to defy 
exact analysis, being an elaborate structure of atoms built up 
into a molecule in which each atom must occupy its true place 
(like every carved stone in a Gothic cathedral), yet it is, as it 
were, only the starting-point or material out of which the infi- 
nitely varied structures of living bodies are formed. The extreme 
mobility and changeability of the structure of these molecules 
enables the protoplasm to be continually modified both in consti- 
tution and form, and, by the substitution or addition of other 
elements, to serve special purposes. Thus, when sulphur in small 
quantities is absorbed and built into the molecular structure, pro- 
teids are formed. These are most abundant in animal structures, 
and give the nourishing properties to meat, cheese, eggs, and 
other animal foods; but they are also found in the vegetable 
kingdom, especially in nuts and seeds such as grain, peas, etc. 
These are generally known as nitrogeneous foods, and are very 
nutritious, but not so easily digestible as meat. Proteids exist 
in very varied forms and often contain phosphorus as well as 
sulphur, but their main characteristic is the large proportion of 
nitrogen they contain, while many other animal and vegetable 
products, as most roots, tubers, and grains, and even fats and 
oils, are mainly composed of starch and sugar. In its chemical 
and physiological aspects protein is thus described by Professor 
W. D. Haliburton : " Proteids are produced only in the living 
laboratory of animals and plants; proteid matter is the all-im- 



CHARACTERS OF LIVING ORGANISM 199 

portant material present in protoplasm. This molecule is the 
most complex that is known; it always contains five and often 
six or even seven elements. The task of thoroughly understand- 
ing its composition is necessarily vast, and advance slow. But, 
little by little, the puzzle is being solved, and this final conquest 
of organic chemistry, when it does arrive, will furnish physiolo- 
gists with new light on many of the dark places of physiological 
science." x 

What makes protoplasm and its modifications still more mar- 
vellous is the power it possesses of absorbing and moulding a 
number of other elements in various parts of living organisms 
for special uses. Such are silica in the stems of the grass 
family, lime and magnesia in the bones of animals, iron in blood, 
and many others. Besides the four elements constituting proto- 
plasm, most animals and plants contain also in some parts of 
their structure sulphur, phosphorus, chlorine, silicon, sodium, 
potassium, calcium, magnesium, and iron ; while, less frequently, 
fluorine, iodine, bromine, lithium, copper, manganese, and alumin- 
ium are also found in special organs or structures ; and the mole- 
cules of all these are carried by the protoplasmic fluids to the 
places where they are required and built into the living structure, 
with the same precision and for similar ends as brick and stone, 
iron, slate, wood, and glass are each utilised in their proper 
places in any large building. 2 The organism, however, is not 
built, but grows. Every organ, every fibre, cell, or tissue is 

1 Address to the British Association, 1902, Section Physiology. 

2 This enumeration of the elements that enter into the structure of plants 
and animals is taken from Professor F. J. Allen's paper already referred to. 



200 MAN'S PLACE IN THE UNIVERSE 

formed from diverse materials, which are first decomposed into 
their elementary molecules, transformed by the protoplasm or by 
special solvents formed from it, carried to the places where they 
are needed by the vital fluids, and there built up atom by atom 
or molecule by molecule into the special structures of which they 
are to form a part. 

But even this marvel of growth and repair of every individual 
organism is far surpassed by the greater marvel of reproduc- 
tion. Every living thing of the higher orders arises from a 
single microscopic cell, when fertilised, as it is termed, by the 
absorption of another microscopic cell derived from a different 
individual. These cells are often, even under the highest powers 
of the microscope, hardly distinguishable from other cells which 
occur in all animals and plants and of which their structure is 
built up; yet these special cells begin to grow in a totally dif- 
ferent manner, and instead of forming one particular part of 
the organism, develop inevitably into a complete living thing 
with all the organs, powers, and peculiarities of its parents, so 
as to be recognisably of the same species. If the simple growth 
of the fully formed organism is a mystery, what of this growth 
of thousands of complex organisms each with all its special pecu- 
liarities, yet all arising from minute germs or cells the diverse 
natures of which are wholly indistinguishable by the highest 
powers of the microscope? This, too, is said to be the work of 
protoplasm under the influence of heat and moisture, and modern 
physiologists hope some day to learn " how it is done." It may 
be well here to give the views of a modern writer on this point. 
Referring to a difficulty which had been stated by Clerk-Maxwell 



CHARACTERS OF LIVING ORGANISM 201 

twenty-five years ago, that there was not room in the reproduc- 
tive cell for the millions of molecules needed to serve as the units 
of growth for all the different structures in the body of the 
higher animals, Professor M'Kendrick says : " But to-day, it 
is reasonable from existing data to suppose that the germinal 
vesicle might contain a million of millions of organic molecules. 
Complex arrangements of these molecules suited for the develop- 
ment of all the parts of a highly complicated organism, might 
satisfy all the demands of the theory of heredity. Doubtless 
the germ was a material system through and through. The 
conception of the physicist was, that molecules were in various 
states of movement; and the thinkers were striving toward a 
kinetic theory of molecules and of atoms of solid matter, which 
might be as fruitful as the kinetic theory of gases. There were 
motions atomic and molecular. It was conceivable that the 
peculiarities of vital action might be determined by the kind of 
motion that took place in the molecules of what we call living 
matter. It might be different in kind from some of the motions 
dealt with by physicists. Life is continually being created from 
non-living material — such, at least, is the existing view of growth 
by the assimilation of food. The creation of living matter out 
of non-living may be the transmission to the dead matter of 
molecular motions which are sui generis in form." This is the 
modern physiological view of " how it may be done," and it seems 
hardly more intelligible than the very old theory of the origin 
of stone axes, given by Adrianus Tollius in 1649, and quoted by 
Mr. E. B. Taylor, who says : " He gives drawings of some ordi- 
nary stone axes and hammers, and tells how naturalists say that 



202 MAN'S PLACE IN THE UNIVERSE 

they are generated in the sky by a fulgureous exhalation con- 
globed in a cloud by the circumfixed humour, and are, as it were, 
baked hard by intense heat, and the weapon becomes pointed by 
the damp mixed with it flying from the dry part, and leaving 
the other end denser, but the exhalations press it so hard that it 
breaks through the cloud and makes thunder and lightning. But 
[he says] if this is really the way in which they are generated, 
it is odd they are not round, and that they have holes through 
them. It is hardly to be believed, he thinks." * And so, when 
the physiologists, determined to avoid the assumption of any- 
thing beyond matter and motion in the germ, impute the whole 
development and growth of the elephant or of man from minute 
cells internally alike, by means of " kinds of motion " and the 
" transmission of motions which are sui generis in form," many 
of us will be inclined to say with the old author — " It is hardly 
to be believed, I think." 

This brief statement of the conclusions arrived at by chemists 
and physiologists as to the composition and structure of organ- 
ised living things has been thought advisable, because the non- 
scientific reader has often no conception of the incomparable 
marvel and mystery of the life-processes he has always seen going 
on, silently and almost unnoticed, in the world around him. And 
this is still more the case now that two-thirds of our population 
are crowded into cities where, removed from all the occupations, 
the charms, and the interests of country life, they are driven to 
seek occupation and excitement in the theatre, the music-hall, or 
the tavern. How little do these know what they lose by being 
1 Early History of Mankind, 2d ed., p. 227. 



CHARACTERS OF LIVING ORGANISM 203 

thus shut out from all quiet intercourse with nature ; its soothing 
sights and sounds ; its exquisite beauties of form and colour ; its 
endless mysteries of birth, and life, and death. Most people 
give scientific men credit for much greater knowledge than they 
possess in these matters ; and many educated readers will, I feel 
sure, be surprised to find that even such apparently simple phe- 
nomena as the rise of the sap in trees are not yet completely 
explained. As to the deeper problems of life, and growth, and 
reproduction, though our physiologists have learned an infinite 
amount of curious or instructive facts, they can give us no intel- 
ligible explanation of them. 

The endless complexities and confusing amount of detail in all 
treatises on the physiology of animals and plants, is such that 
the average reader is overwhelmed with the mass of knowledge 
presented to him, and concludes that after such elaborate re- 
searches everything must be known, and that the almost universal 
protests against the need of any causes but the mechanical, physi- 
cal, and chemical laws and forces, are well founded. I have, 
therefore, thought it advisable to present a kind of bird's-eye 
view of the subject, and to show, in the words of the greatest 
living authorities on these matters, both how complex are the 
phenomena and how far our teachers are from being able to give 
us any adequate explanation of them. 

I venture to hope that the very brief sketch of the subject I 
have been able to give will enable my readers to form some faint 
general conception of the infinite complexity of life and the 
various problems connected with it; and that they will thus be 
the better enabled to appreciate the extreme delicacy of those 



204 MAN'S PLACE IN THE UNIVERSE 

adjustments, those forces, and those complex conditions of the 
environment, that alone render life, and above all the grand age- 
long panorama of the development of life, in any way possible. 
It is to these conditions, as they prevail in the world around us, 
that we will now direct our attention. 



CHAPTER XI 

THE PHYSICAL CONDITIONS ESSENTIAL FOR ORGANIC LIFE 

The physical conditions on the surface of our earth which 
appear to be necessary for the development and maintenance 
of living organisms may be dealt with under the following 
headings : 

1. Regularity of heat-supply, resulting in a limited range 
of temperature. 

2. A sufficient amount of solar light and heat. 

3. Water in great abundance, and universally distributed. 

4. An atmosphere of sufficient density, and consisting of the 
gases which are essential for vegetable and animal life. These 
are Oxygen, Carbonic-acid gas, Aqueous vapour, Nitrogen, 
and Ammonia. These must all be present in suitable pro- 
portions. 

5. Alternations of day and night. 

SMALL RANGE OF TEMPERATURE REQUIRED FOR 
GROWTH AND DEVELOPMENT 

Vital phenomena for the most part occur between the tempera- 
tures of freezing water and 104° Fahr., and this is supposed to 
be due mainly to the properties of nitrogen and its compounds, 

205 



206 MAN'S PLACE IN THE UNIVERSE 

which, between these temperatures only, can maintain those pecu- 
liarities which are essential to life — extreme sensitiveness and 
lability ; facility of change as regards chemical combination and 
energy; and other properties which alone render nutrition, 
growth, and continual repair possible. A very small increase 
or decrease of temperature beyond these limits, if continued for 
any considerable time, would certainly destroy most existing 
forms of life, and would not improbably render any further 
development of life impossible except in some of its lowest 
forms. 

As one example of the direct effects of increased temperature, 
we may adduce the coagulation of albumen. This substance is 
one of the proteids, and plays an important part in the vital 
phenomena of both plants and animals, and its fluidity and power 
of easy combination and change of form are lost by any degree 
of coagulation which takes place at about 160° Fahr. 

The extreme importance to all the higher organisms of a 
moderate temperature is strikingly shown by the complex and 
successful arrangements for maintaining a uniform degree of 
heat in the interior of the body. The normal blood-heat in a 
man is 98° Fahr., and this is constantly maintained within one 
or two degrees though the external temperature may be more 
than fifty degrees below the freezing-point. High temperatures 
upon the earth's surface do not range so far from the mean as 
do the low. In the greater part of the tropics the air-tempera- 
ture seldom reaches 96° Fahr., though in arid districts and 
deserts, which occur chiefly along the margins of the northern 
and southern tropics, it not unfrequently surpasses 110° Fahr., 



ESSENTIAL LIFE-CONDITIONS 207 

and even occasionally rises to 115° or 120° in Australia and 
Central India. Yet with suitable food and moderate care the 
blood-temperature of a healthy man would not rise or fall more 
than one or at most two degrees. The great importance of this 
uniformity of temperature in all the vital organs is distinctly 
shown by the fact that when, during fevers, the temperature 
of the patient rises six degrees above the normal amount, his con- 
dition is critical, while an increase of seven or eight degrees is 
an almost certain indication of a fatal result. Even in the 
vegetable kingdom seeds will not germinate under a temperature 
of four or five degrees above the freezing-point. 

Now this extreme sensibility to variations of internal tempera- 
ture is quite intelligible when we consider the complexity and 
instability of protoplasm, and of all the proteids in the living 
organism, and how important it is that the processes of nutrition 
and growth, involving constant motion of fluids and incessant 
molecular decompositions and recombinations, should be effected 
with the greatest regularity. And though a few of the higher 
animals, including man, are so perfectly organised that they can 
adapt or protect themselves so as to be able to live under very 
extreme conditions as regards temperature, yet this is not the 
case with the great majority, or with the lower types, as evi- 
denced by the almost complete absence of reptiles from the Arctic 
regions. 

It must also be remembered that extreme cold and extreme 
heat are nowhere perpetual. There is always some diversity of 
seasons, and there is no land animal which passes its whole life 
where the temperature never rises above the freezing point. 



208 MAN'S PLACE IN THE UNIVERSE 

THE NECESSITY OF SOLAR LIGHT 

Whether the higher animals and man could have been developed 
upon the earth without solar light, even if all the other essential 
conditions were present, is doubtful. That, however, is not the 
point I am at present considering, but one that is much more 
fundamental. Without plant life land animals at all events could 
never have come into existence, because they have not the power 
of making protoplasm out of inorganic matter. The plant alone 
can take the carbon out of the small proportion of carbonic acid 
in the atmosphere, and with it and the other necessary elements, 
as already described, build up those wonderful carbon compounds 
which are the very foundation of animal life. But it does this 
solely by the agency of solar light, and even uses a special portion 
of that light. Not only, therefore, is a sun needed to give light 
and heat, but it is quite possible that any sun would not answer 
the purpose. A sun is required whose light possesses those spe- 
cial rays which are effective for this operation, and as we know 
that the stars differ greatly in their spectra, and therefore in 
the nature of their light, all might not be able to effect this great 
transformation, which is one of the very first steps in rendering 
animal life possible on our earth, and therefore probably on all 
earths. 

WATER A FIRST ESSENTIAL OF ORGANIC LIFE 

It is hardly necessary to point out the absolute necessity of 
water, since it actually constitutes a very large proportion of 
the material of every living organism, and about three-fourths 



ESSENTIAL LIFE-CONDITIONS 209 

of our own bodies. Water, therefore, must be present every- 
where, in one form or another, on any globe where life is pos- 
sible. Neither animal nor plant can exist without it. It must 
also be present in such quantity and so distributed as to be 
constantly available on every part of a globe where life is to be 
maintained ; and it is equally necessary that it should have per- 
sisted in equal profusion throughout those enormous geological 
epochs during which life has been developing. We shall see 
later on how very special are the conditions that have secured 
this continuous distribution of water on our earth, and we shall 
also learn that this large amount of water, its wide distribution, 
and its arrangement with regard to the land-surface, is an essen- 
tial factor in producing that limited range of temperature which, 
as we have seen, is a primary condition for the development and 
maintenance of life. 

THE ATMOSPHERE MUST BE OF SUFFICIENT DENSITY 
AND COMPOSED OF SUITABLE GASES 

The atmosphere of any planet on which life can be developed must 
have several qualities which are unconnected with each other, and 
the coincidence of which may be a rare phenomenon in the uni- 
verse. The first of these is a sufficient density, which is required 
for two purposes — as a storer of heat, and in order to supply 
the oxygen, carbonic acid, and aqueous vapour in sufficient 
quantities for the requirements of vegetable and animal life. 

As a reservoir of heat and a regulator of temperature, a rather 
dense atmosphere is a first necessity, in co-operation with the 



210 MAN'S PLACE IN THE UNIVERSE 

large quantity and wide distribution of water referred to in the 
last section. The very different character of our southwest from 
out northeast winds is a good illustration of its power of dis- 
tributing heat and moisture. This it does owing to the peculiar 
property it possesses of allowing the sun's rays to pass freely 
through it to the earth which it warms, but acting like a blanket 
in preventing the rapid escape of the non-luminous heat so pro- 
duced. But the heat stored up during the day is given out at 
night, and thus secures a uniformity of temperature which would 
not otherwise exist. This effect is strikingly seen at high alti- 
tudes, where the temperature becomes lower and lower, till at 
a not very great elevation, even in the tropics, snow lies on the 
ground all the year round. This is almost wholly due to the 
rarity of the air, which, on that account, has not so much 
capacity for heat, ft also allows the heat it acquires to radiate 
more freely than denser air, so that the nights are much colder. 
At about 18,000 feet high our atmosphere is exactly half its 
density at the sea-level. This is considerably higher than the 
usual snow-line, even under the equator, whence it follows that 
if our atmosphere were only half its present density it would 
render the earth unsuitable for the higher forms of animal life. 
It is not easy to say exactly what would be the result as regards 
climate ; but it seems likely that, except perhaps in limited areas 
in the tropics, where conditions were very favourable, the whole 
land-surface would become buried in snow and ice. This appears 
inevitable, because evaporation from the oceans by direct sun- 
heat would be more rapid than now ; but as the vapour rose in the 
rare atmosphere it would rapidly become frozen, and snow would 



ESSENTIAL LIFE-CONDITIONS 211 

fall almost perpetually, although it might not lie permanently 
on the ground in the equatorial lowlands. It appears certain, 
therefore, that with half our present bulk of atmosphere life 
would be hardly possible on the earth on account of lowered tem- 
perature alone. And as there would certainly be an added diffi- 
culty in the needful supply of oxygen to animals and carbonic 
acid to plants, it seems highly probable that a reduction of 
density of even one-fourth might be sufficient to render a large 
portion of the globe a snow-and-ice-clad waste, and the remainder 
liable to such extremes of climate that only low forms of life 
could have arisen and been permanently maintained. 

THE GASES OF THE ATMOSPHERE 

Coming now to consider the constituent gases of the atmosphere, 
there is reason to believe that they form a mixture as nicely bal- 
anced in regard to animal and vegetable life as are the density 
and the temperature. At a first view of the subject we might 
conclude that oxygen is the one great essential for animal life, 
and that all else is of little importance. But further considera- 
tion shows us that nitrogen, although merely a diluent of the 
oxygen as regards the respiration of animals, is of the first im- 
portance to plants, which obtain it from the ammonia formed in 
the atmosphere and carried down into the soil by the rain. Al- 
though there is only one part of ammonia to a million of air, yet 
upon this minute proportion the very existence of the animal 
world depends, because neither animals nor plants can assimilate 
the free nitrogen of the air into their tissues. 



212 MAN'S PLACE IN THE UNIVERSE 

Another fundamentally important gas in the atmosphere is 
carbonic acid, which forms about four parts in ten thousand 
parts of air, and, as already stated, is the source from which 
plants build up the great bulk of their tissues, as well as those 
protoplasms and proteids so absolutely necessary as food for 
animals. An important fact to notice here is, that carbonic 
acid, so essential to plants, and to animals through plants, is yet 
a poison to animals. When present in much more than the 
normal quantity, as it often is in cities and in badly ventilated 
buildings, it becomes highly prejudicial to health; but this is 
believed to be partly due to the various corporeal emanations 
and other impurities associated with it. Pure carbonic acid gas 
to the amount of even one per cent, in otherwise pure air, can, 
it is said, be breathed for a time without bad effects, but any- 
thing more than this proportion will soon produce suffocation. 
It is probable, therefore, that a very much smaller proportion 
than one per cent., if constantly present, would be dangerous 
to life ; though no doubt, if this had always been the proportion, 
life might have been developed in adaptation to it. Considering, 
however, that this poisonous gas is largely given out by the 
higher animals as a product of respiration, it would evidently be 
dangerous to the permanence of life if the quantity forming a 
constant constituent of the atmosphere were much greater than 
it is. 

AQUEOUS VAPOUR IN THE ATMOSPHERE 

This water-gas, although it occurs in the atmosphere in largely 
varying quantities, is yet, in two distinct ways, essential to or- 



ESSENTIAL LIFE-CONDITIONS 213 

ganic life. It prevents the too rapid loss of moisture from the 
leaves of plants when exposed to the sun, and it is also absorbed 
by the upper surface of the leaf and by the young shoots, which 
thus obtain both water and minute quantities of ammonia when 
the supply by the roots is insufficient. But it is of even more 
vital importance in supplying the hydrogen which, when united 
with the nitrogen of the atmosphere by electrical discharges, 
produces the ammonia, which is the main source of all the pro- 
teids of the plant, which proteids are the very foundation of 
animal life. 

From this brief statement of the purposes served by the 
various gases forming our atmosphere, we see that they are to 
some extent antagonistic, and that any considerable increase of 
one or the other would lead to results that might be injurious 
either directly or in their ultimate results. And as the elements 
which constitute the bulk of all living matter possess properties 
which render them alone suitable for the purpose, we may con- 
clude that the proportions in which they exist in our atmosphere 
cannot be very widely departed from wherever organic forms are 
developed. 

THE ALTERNATION OF DAY AND NIGHT 

Although it is difficult to decide positively whether alternations 
of light and darkness at short intervals are absolutely essential 
for the development of the various higher forms of life, or 
whether a world in which light was constant might do as well, 
yet on the whole it seems probable that day and night are really 
important factors. All nature is full of rhythmic movements 



214 MAN'S PLACE IN THE UNIVERSE 

of infinitely varied kinds, degrees, and durations. All the mo- 
tions and functions of living things are periodic; growth and 
repair, assimilation and waste, go on alternately. All our organs 
are subject to fatigue and require rest. All kinds of stimulus 
must be of short duration or injurious results follow. Hence 
the advantage of darkness, when the stimuli of light and 
heat are partially removed, and we welcome " Tired nature's 
sweet restorer, balmy sleep " — giving rest to all the senses 
and faculties of body and mind, and endowing us with 
renewed vigour for another period of activity and enjoyment 
of life. 

Plants as well as animals are invigorated by this nightly 
repose; and all alike benefit by these longer periods of greater 
and less amounts of work caused by summer and winter, dry and 
wet seasons. It is a suggestive fact, that where the influence 
of heat and light is greatest — within the tropics — the days and 
nights are of equal length, giving equal periods of activity and 
rest. But in cold and Arctic regions where, during the short 
summer, light is nearly perpetual, and all the functions of life, 
in vegetation especially, go on with extreme rapidity, this is fol- 
lowed by the long rest of winter, with its short days and greatly 
lengthened periods of darkness. 

Of course, all this is rather suggestion than proof. It is pos- 
sible that in a world of perpetual day or in one of perpetual 
night, life might have been developed. But on the other hand, 
considering the great variety of physical conditions which are 
seen to be necessary for the development and preservation of life 
in its endless varieties, any prejudicial influences, however slight, 



ESSENTIAL LIFE-CONDITIONS 215 

might turn the scale, and prevent that harmonious and continu- 
ous evolution which we know must have occurred. 

So far I have only considered the question of day and night 
as regards the presence or absence of light. But it is probably 
far more important in its heat aspect; and here its period be- 
comes of great, perhaps vital, importance. With its present 
duration of twelve hours day and twelve night on the average, 
there is not time, even between the tropics, for the earth to be- 
come so excessively heated as to be inimical to life ; while a con- 
siderable portion of the heat, stored up in the soil, the water, 
and the atmosphere, is given out at night, and thus prevents a 
too sudden and injurious contrast of heat and cold. If the 
day and night were each very much longer — say 50 or 100 
hours — it is quite certain that during a day of that dura- 
tion, the heat would become so great as to be inimical, perhaps 
prohibitive, to most forms of life; while the absence of all sun- 
heat for an equally long period would result in a temperature 
far below the freezing point of water. It is doubtful whether 
any high forms of animal life could have arisen under such great 
and continual contrasts of temperature. 

We will now proceed to point out the special features which, in 
our earth, have combined to bring about and to maintain the 
various and complex conditions we have seen to be essential for 
life as it exists around us. 



CHAPTER XII 

THE EARTH IN ITS RELATION TO THE DEVELOPMENT 
AND MAINTENANCE OF LIFE 

The first circumstance to be considered in relation to the habi- 
tability of a planet is its distance from the sun. We know that 
the heating power of the sun upon our earth is ample for the 
development of life in an almost infinite variety of forms; and 
we have a large amount of evidence to show that, were it not 
for the equalising power of air and water, distributed as they 
are with us, the heat received from the sun would be sometimes 
too great and sometimes too little. In some parts of Africa, 
Australia, and India, the sandy soil becomes so hot that an egg 
can be cooked by placing it just below the surface. On the other 
hand, at an elevation of about 12,000 feet in lat. 40° it freezes 
every night, and throughout the day in all places sheltered from 
the sun. Now, both these temperatures are adverse to life, and 
if either of them persisted over a considerable portion of the 
earth, the development of life would have been impossible. But 
the heat derived from the sun is inversely as the square of the 
distance, so that at half the distance we should have four times 
as much heat, and at twice the distance only one-fourth of the 
heat. Even at two-thirds of the distance we should receive more 
than twice as much heat ; and, considering the facts as to the ex- 

216 



THE EARTH, IN RELATION TO LIFE 217 

treme sensitiveness of protoplasm and the coagulation of albu- 
men, it seems certain that we are situated in what has been termed 
the temperate zone of the solar system, and that we could not 
be removed far from our present position without endangering 
a considerable portion of the life now existing upon the earth, 
and in all probability rendering the actual development of life 
through all its phases and gradations, impossible. 

THE OBLIQUITY OF THE ECLIPTIC 

The effect of the obliquity of the earth's equator to its path 
round the sun, upon which depend our varying seasons and the 
inequality of day and night throughout all the temperate zones, 
is very generally known. But it is not usually considered that 
this obliquity is of any great importance as regards the suitabil- 
ity of the earth for the development and maintenance of life; 
and it seems to have been passed over as an accident hardly worth 
notice, because almost any other obliquity or none at all would 
have been equally advantageous. But if we consider what the 
direction of the earth's axis might possibly have been, we shall 
find that it is really a matter of great importance from our 
present point of view. 

Let us suppose, first, that the earth's axis was, like that of 
Uranus, almost exactly in the plane of its orbit or directed 
towards the sun. There can be little doubt that such a posi- 
tion would have rendered our world unfitted for the development 
of life. For the result would be the most tremendous contrasts 
of the seasons ; at midwinter, on one half the globe, arctic night 



218 MAN'S PLACE IN THE UNIVERSE 

and more than arctic cold would prevail ; while on the other half 
there would be a midsummer of continuous day with a vertical 
sun and such an amount of heat as nowhere exists with us. At 
the two equinoxes the whole globe would enjoy equal day and 
night, all our present tropics and part of the sub-tropical zone 
having the sun at noon so near to the zenith as to have the essen- 
tial of a tropical climate. But the change to about a month of 
constant sunshine or a month of continuous night would be so 
rapid, that it seems almost impossible that either vegetable or 
animal life would ever have developed under such terrible condi- 
tions. 

The other extreme direction of the earth's axis, exactly at 
right angles to the plane of the orbit, would be much more 
favourable, but would still have its disadvantages. The whole 
surface from equator to poles would enjoy equal day and night, 
and every part would receive the same amount of sun-heat all 
the year round, so that there would be no change of seasons ; but 
the heat received would vary with the latitude. In our latitude 
the sun's altitude at noon all the year would be less than 40°, the 
same as now occurs at the equinoxes, and we might therefore 
have a perpetual spring as regards temperature. But the con- 
stancy of the heat in the equatorial and tropical regions and of 
cold towards the poles would lead to a more constant and more 
rapid circulation of air, and we should probably experience such 
continuous northwesterly winds as to render our climate always 
cold and probably very damp. Near the poles the sun would 
always be on, or close to, the horizon, and would give so little 
heat that the sea might be perpetually frozen and the land 



THE EARTH, IN RELATION TO LIFE 219 
deeply snow-buried; and these conditions would probably extend 
into the temperate zone, and possibly so far south as to render 
life impossible in our latitudes, since whatever results arose would 
be due to permanent causes, and we know how powerful are snow 
and ice to extend their sway over adjacent areas if not counter- 
acted by summer heat or warm moist winds. On the whole, 
therefore, it seems probable that this position of the earth's axis 
would result in a much smaller portion of its surface being 
capable of supporting a luxuriant and varied vegetable and ani- 
mal life than is now the case; while the extreme uniformity of 
conditions everywhere present might be so antagonistic to the 
great law of rhythm that seems to pervade the universe, and 
be in other ways so unfavourable, that life-development would 
probably have taken quite a different course from that which it 
has taken. 

It appears almost certain, therefore, that some intermediate 
position of the axis would be the most favourable ; and that 
which actually exists seems to combine the advantage of change 
of seasons with good climatical conditions over the largest pos- 
sible area. We know that during the greater part of the epoch 
of life-development this area was much greater than at present, 
since a luxuriant vegetation of deciduous and evergreen trees 
and shrubs extended up to and within the Arctic Circle, leading 
to the formation of coal-beds both in palaeozoic and tertiary 
times ; the extremely favourable conditions for organic life which 
then prevailed over so large a portion of the globe's surface, 
and which persisted down to a comparatively recent epoch, lead 
to the conclusion that no more favourable degree of obliquity 



220 MAN'S PLACE IN THE UNIVERSE 

was possible than that which we actually possess. A short ac- 
count of the evidence on this interesting subject will now be 
given. 

PERSISTENCE OF MILD CLIMATE THROUGH GEOLOGICAL TIME 

The whole of the geological evidence goes to show that in remote 
ages the climate of the earth was generally more uniform, though 
perhaps not warmer, than it is now, and this can be best explained 
by a slightly different distribution of sea and land, which allowed 
the warm waters of the tropical oceans to penetrate into various 
parts of the continents (which were then more broken up than 
they are now), and also to extend more freely into the Arctic 
regions. So soon as we go back into the tertiary period, we find 
indications of a warmer climate in the north temperate zone ; and 
when we reach the middle of that period, we find abundant indica- 
tions, both in plant and animal remains, of mild climates near to 
the Arctic Circle, or actually within it. 

On the west coast of Greenland, in 70° N. lat., there are found 
abundance of fossil plants very beautifully preserved, among 
which are many different species of oaks, beeches, poplars, plane- 
trees, vines, walnuts, plums, chestnuts, sequoias, and numerous 
shrubs — 137 species in all, indicating a vegetation such as now 
grows in the north temperate parts of America and Eastern 
Asia. And even further north, in Spitzbergen, in N. lat. 
78° and 79°, a somewhat similar flora is found, not quite so 
varied, but with oaks, poplars, birches, planes, limes, hazels, 
pines, and many aquatic plants such as may now be found in West 
Norway and in Alaska, nearly twenty degrees further south. 



THE EARTH, IN RELATION TO LIFE 221 

Still more remote, in the Cretaceous period, fossil plants have 
been found in Greenland, consisting of ferns, cycads, conifers, 
and such trees and shrubs as poplars, sassafras, andromedas, 
magnolias, myrtles, and many others, similar in character and 
often identical in species with fossils of the same period found 
in Central Europe and the United States, indicating a widespread 
uniformity of climate, such as would be brought about by the 
great ocean-currents carrying the warm waters of the tropics 
into the Arctic seas. 

Still further back, in the Jurassic period, we have proofs of a 
mild climate in East Siberia and at Ando in Norway just within 
the Arctic Circle, in numerous plant remains, and also remains 
of great reptiles allied to those found in the same strata in all 
parts of the world. Similar phenomena occur in the still earlier 
Triassic period; but we will pass on to the much more remote 
Carboniferous period, during which most of the great coal-beds 
of the world were formed from a luxuriant vegetation, consisting 
mostly of ferns, giant horse-tails, and primitive conifers. The 
luxuriance of these plants, which are often found beautifully 
preserved and in immense quantities, is supposed to indicate an 
atmosphere in which carbonic acid gas was much more abundant 
than now; and this is rendered probable by the small number 
and low type of terrestrial animals, consisting of a few insects 
and amphibia. 

But the interesting point is, that true coal-beds, with similar 
fossils to those of our own coal-measures, are found at Spitz- 
bergen and at Bear Island in East Siberia, both far within the 
Arctic Circle, again indicating a great uniformity of climate, 



222 MAN'S PLACE IN THE UNIVERSE 

and probably a denser and more vapour-laden atmosphere, which 
would act as a blanket over the earth and preserve the heat 
brought to the Arctic seas by the ocean currents from the warmer 
regions. 

The still earlier silurian rocks are also found abundantly in 
the Arctic regions, but their fossils are entirely of marine ani- 
mals. Yet they show the same phenomena as regards cli- 
mate, since the corals and cephalopodous mollusca found in the 
Arctic beds closely resemble those of all other parts of the 
earth. 1 

Many other facts indicate that throughout the enormous 
periods required for the development of the varied forms of life 
upon the earth, the great phenomena of nature were but little 
different from those that prevail in our own times. The slow 
and gentle processes by which the various vegetable and animal 
remains were preserved are shown by the perfect state in which 
many of the fossils exist. Often trunks of trees, cycads, and 
tree-ferns are found standing erect, with their roots still im- 
bedded in the soil they grew in. Large leaves of poplars, maples, 
oaks, and other trees are often preserved in as perfect a state as 
if gathered by a botanist and dried between paper for his 
herbarium, and the same is especially the case with the beautiful 
ferns of the Permian and Carboniferous periods. Throughout 
these and most other formations well-preserved ripple-marks are 
found in the solidified mud or sand of old seashores, differing in 

1 For a fuller account of this Arctic fauna and flora see the works of Sir 
C. Lyell, Sir A. Geikie, and other geologists. A full summary of it is also 
given in the author's Island Life. 



THE EARTH, IN RELATION TO LIFE 223 

no respect from similar marks to be found on almost every coast 
to-day. Equally interesting are the marks of rain-drops pre- 
served in the rocks of almost all ages, and Sir Charles Lyell has 
given illustrations of recent impressions of raindrops on the ex- 
tensive mud-flats of Nova Scotia, and also an illustration of 
rain-drops on a slab of shale from the carboniferous formation 
of the same country ; and the two are as much alike as the prints 
of two different showers a few days apart. The general size 
and form of the drops are almost identical, and imply a great 
similarity in the general atmospheric conditions. 

We must not forget that this presence of rain throughout 
geological time implies, as we have seen in our last chapter, a 
constant and universal distribution of atmospheric dust. The 
two chief sources of this dust — the total quantity of which in the 
atmosphere must be enormous — are volcanoes and deserts, and 
we are therefore sure that these two great natural phenomena 
have always been present. Of volcanoes we have ample inde- 
pendent evidence in the presence of lavas and volcanic ashes, as 
well as actual stumps or cores of old volcanoes, through all geo- 
logical formations ; and we can have little doubt that deserts also 
were present, though perhaps not always so extensive as they 
are now. It is a very suggestive fact that these two phenomena, 
usually held to be blots on the fair face of nature, and even to 
be opposed to belief in a beneficent Creator, should now be proved 
to be really essential to the earth's habitability. 

Notwithstanding this prevalence of warm and uniform condi- 
tions, there is also evidence of considerable changes of climate; 
and at two periods — in the Eocene and in the remote Permian — 



224 MAN'S PLACE IN THE UNIVERSE 

there are even indications of ice-action, so that some geologists 
believe that there were then actual glacial epochs. But it seems 
more probable that they imply only local glaciation, owing to 
there having been high land and other suitable conditions for 
the production of glaciers in certain areas. 

The whole bearing of the geological evidence indicates the 
wonderful continuity of conditions favourable for life, and for 
the most part of climatal conditions more favourable than those 
now prevailing, since a larger extent of land towards the North 
Pole was available for an abundant vegetation, and in all prob- 
ability for an equally abundant animal life. We know, too, that 
there was never any total break in life-development ; no epoch 
of such lowering or raising of temperature as to destroy all life ; 
no such general subsidence as to submerge the whole land-surface. 
Although the geological record is in parts very imperfect, yet 
it is, on the whole, wonderfully complete ; and it presents to our 
view a continuous progress, from simple to complex, from lower 
to higher. Type after type becomes highly specialised in adap- 
tation to local or climatal conditions, and then dies out, giving 
room for some other type to arise and be specialised in harmony 
with the changed conditions. The general character of the in- 
organic change appears to have been from more insular to more 
continental conditions, accompanied by a change from more uni- 
form to less uniform climates, from an almost sub-tropical 
warmth and moisture, extending up to the Arctic Circle, to that 
diversity of tropical, temperate, and cold areas, capable of sup- 
porting the greatest possible variety in the forms of life, and 
which seems especially adapted to stimulate mankind to civilisa- 



THE EARTH, IN RELATION TO LIFE 225 

tion and social development by means of the necessary struggle 
against, and utilisation of, the various forces of nature. 

WATEE, ITS AMOUNT AND DISTRIBUTION ON THE EARTH 

Although it is generally known that the oceans occupy more 
than two-thirds of the whole surface of the globe, the enormous 
bulk of the water in proportion to the land that rises above its 
surface is hardly ever appreciated. But as this is a matter of 
the greatest importance, both as regards the geological history 
of the globe and the special subj ect we are here discussing, it will 
be necessary to enter into some details in regard to it. 

According to the best recent estimates, the land area of the 
globe is 0.28 of the whole surface, and the water area 0.72. But 
the mean height of the land above the sea-level is found to be 
2250 feet, while the mean depth of the seas and oceans is 13,860 
feet ; so that though the water area is two and a half times that 
of the land, the mean depth of the water is more than six times 
the mean height of the land. This is, of course, due to the fact 
that lowlands occupy most of the land-area, the plateaus and 
high mountains a comparatively small portion of it; while, 
though the greatest depths of the oceans about equal the greatest 
heights of the mountains, yet over enormous areas the oceans are 
deep enough to submerge all the mountains of Europe and tem- 
perate North America, except the extreme summits of one or two 
of them. Hence it follows that the bulk of the oceans, even 
omitting all the shallow seas, is more than thirteen times that 
of the land above sea-level ; and if all the land surface and ocean 



226 MAN'S PLACE IN THE UNIVERSE 

floors were reduced to one level, that is, if the solid mass of the 
globe were a true oblate spheroid, the whole would be covered with 
water about two miles deep. The diagram here given will render 
this more intelligible and will serve to illustrate what follows. 

DIAGRAM OF PROPORTIONATE MEAN HEIGHT OF LAND AND 
DEPTH OF OCEANS 




Oceax Area — .72 of area of Globe. 

In this diagram the lengths of the sections representing land 
and ocean are proportionate to their areas, while the thickness 
of each is proportionate to their mean height and mean depth 
respectively. Hence the two sections are in correct proportion 
to their cubic contents. 

A mere inspection of this diagram is sufficient to disprove the 
old idea, still held by a few geologists and by many biologists, 
that oceans and continents have repeatedly changed places during 
geological times, or that the great oceans have again and again 
been bridged over to facilitate the distribution of beetles or birds, 
reptiles or mammals. We must remember that although the 
diagram shows the continents and oceans as a whole, yet it also 
shows, with quite sufficient accuracy, the proportions of each of 
the great continents to the oceans which are adjacent to them. 
It must also be borne in mind that there can be no elevation on a 
large scale without a corresponding subsidence elsewhere; be- 



THE EARTH, IN RELATION TO LIFE 227 

cause if there were not, a vast unsupported hollow would be left 
beneath the rising land or in some part adjacent to it. 

Now, looking at the diagram and at a chart or globe, try to 
imagine the ocean bottom rising gradually, to form a continent 
joining Africa with South America or with Australia (both of 
which are demanded by many biologists ) : it is clear that, while 
such an elevation was going on, either some continental land or 
some other part of the ocean-bed must sink to a corresponding 
amount. We shall then see, that if such changes of elevation 
on a continental scale have taken place again and again at dif- 
ferent periods, it would have been almost impossible, on every 
occasion, to avoid a whole continent being submerged (or even all 
the continents ) in order to equalise subsidence with elevation while 
new continents were being raised up from the abyssal depths of 
the ocean. We conclude, therefore, that with the exception of a 
comparatively narrow belt around the continents, which may 
be roughly indicated by the thousand fathom line of soundings, 
the great ocean depths are permanent features of the earth's 
surface. It is this stability of the general distribution of land 
and water that has secured the continuity of life upon the earth. 
Had the great oceanic basins, on the other hand, been unstable, 
changing places with the land at various periods of geological 
time, they would, almost certainly, again and again have swal- 
lowed up the land in their vast abysses, and have thus destroyed 
all the organic life of the world. 

There are many confirmatory proofs of this view (which is now 
widely accepted by geologists and physicists), and a few of them 
may be briefly stated. 



228 MAN'S PLACE IN THE UNIVERSE 

1. None of the continents present us with marine deposits of 
any one geological age and occupying a large part of the surface 
of each, as must have been the case had they ever been sunk deep 
beneath the ocean and again elevated ; neither do any of them con- 
tain extensive formations corresponding to the deep oceanic clays 
and oozes, which again they must have done had they been at any 
time raised up from the ocean depths. 

% All the continents present an almost complete and continu- 
ous series of rocks of all geological ages, and in each of the great 
geological periods there are found fresh water and estuarine 
deposits, and even old land surfaces, demonstrating continuity 
of continental or insular conditions. 

3. All the great oceans possess, scattered over them, a few or 
many islands termed " oceanic," and characterised by a volcanic 
or coraline structure, with no ancient stratified rocks in any one 
of them ; and in none of these is there found a single indigenous 
land mammal or amphibian. It is incredible that, if these oceans 
had ever contained extensive continents, and if these oceanic 
islands are — as even now they are often alleged to be — parts 
of these now submerged continents, no one fragment of any of 
the old stratified rocks, which characterise all existing continents, 
should remain to show their origin. In the Atlantic we find the 
Azores, Madeira, and St. Helena; in the Indian Ocean, Mau- 
ritius, Bourbon, and Kerguelen Island; in the Pacific, the Fiji, 
Samoan, Society, Sandwich, and Galapagos Islands, all without 
exception telling us the same tale, that they have been built up 
from the ocean depths by submarine volcanoes and coralline 
growths, but have never formed part of continental areas. 



THE EARTH, IN RELATION TO LIFE 229 

4. The contours of the floors of all the great oceans, now 
fairly well known through the soundings of exploring vessels 
and for submarine telegraph lines, also give confirmatory evi- 
dence that they have never been continental land. For if any 
part of them were a sunken continent, that part must have re- 
tained some impress of its origin. Some of the numerous moun- 
tain ranges which characterise every continent would have re- 
mained. We should find slopes of from 20° to 50° not uncom- 
mon, while valleys bordered by rocky precipices, as in Lake Lu- 
cerne and a hundred others, or isolated rock-walled mountains like 
Roraima, or ranges of precipices as in the Ghats of India or the 
Fiords of Norway, would frequently be met with. But not a 
single feature of this kind has ever been found in the ocean 
abysses. Instead of these we have vast plains which, if the water 
were removed, would appear almost exactly level, with no abrupt 
slopes anywhere. When we consider that deposits from the land 
never reach these remote ocean depths, and that there is no wave 
action below a few hundred feet, these continental features once 
submerged would be indestructible; and their total absence is, 
therefore, itself a demonstration that none of the great oceans 
are on the sites of submerged continents. 

HOW OCEAN DEPTHS WERE PRODUCED 

It is a very difficult problem to determine how the vast basins 
which are filled by the great oceans, especially that of the 
Pacific, were first produced. When the earth's surface was still 
in a molten state, it would necessarily take the form of a true 



230 MAN'S PLACE IN THE UNIVERSE 

oblate spheroid, with a compression at the poles due to its speed 
of rotation, which is supposed to have been very great. The 
crust formed by the gradual cooling of such a globe would be of 
the same general form, and, being thin, would easily be fractured 
or bent so as to accommodate itself to any unequal stresses from 
the interior. As the crust thickened and the whole mass slowly 
cooled and contracted, fissures and crumpling would occur, the 
former serving as outlets for volcanic activities whose results are 
found throughout all geological ages ; the latter producing moun- 
tain chains in which the rocks are almost always curved, folded, 
or even thrust over each other, indicating the mighty forces due 
to the adjustments of a solid crust upon a shrinking fluid or 
semi-fluid interior. 

But during this whole process there seem to be no forces at 
work that could lead to the production of such a feature as the 
Pacific, a vast depression covering nearly one-third of the whole 
surface of the globe. The Atlantic Ocean, being smaller and 
nearly opposite to the Pacific, but approximately of equal depth, 
may be looked upon as a complementary phenomenon which will 
be probably explained as a result of the same causes as the vaster 
cavity. 

So far as I am aware, there is only one suggested cause of the 
formation of these great oceans that seems adequate ; and as that 
cause is to some extent supported by quite independent astronomi- 
cal evidence, and also directly bears upon the main subject of 
the present volume, it must be briefly considered. 

A few years ago, Professor George Darwin, of Cambridge, 
arrived at a certain conclusion as to the origin of the moon, which 



THE EARTH, IN RELATION TO LIFE 231 

is now comparatively well known by Sir Robert Ball's popular 
account of it in his small volume, Time and Tide. Briefly stated, 
it is as follows: The tides produce friction on the earth and 
very slowly increase the length of our day, and also cause the 
moon to recede further from us. The day is lengthened only by 
a small fraction of a second in a thousand years, and the moon is 
receding at an equally imperceptible rate. But as these forces are 
constant, and have always acted on the earth and moon, as we go 
back and back into the almost infinite past we come to a time when 
the rotation of the earth was so rapid that gravity at the equator 
could hardly retain its outer portion, which was spread out so 
that the form of the whole mass was something like a cheese with 
rounded edges. And about the same epoch the distance of the 
moon is found to have been so small that it was actually touching 
the earth. All this is the result of mathematical calculation 
from the known laws of gravitation and tidal effects; and as it 
is difficult to see how so large a body as the moon could have 
originated in any other way, it is supposed that at a still earlier 
period the moon and earth were one, and that the moon separated 
from the parent mass owing to centrifugal force generated by 
the earth's rapid rotation. Whether the earth was liquid or 
solid at this epoch, and exactly how the separation occurred, is 
not explained either by Professor Darwin or Sir Robert Ball; 
but it is a very suggestive fact that, quite recently, it has been 
shown, by means of the spectroscope, that double stars of short 
period do originate in this way from a single star, as already 
described in our sixth chapter ; but in these cases it seems prob- 
able that the parent star is in a gaseous state. 



232 MAN'S PLACE IN THE UNIVERSE 

These investigations of Professor G. Darwin have been made 
use of by the Rev. Osmond Fisher (in his very interesting and 
important work, Physics of the Earth's Crust) to account for 
the basins of the great oceans, the Pacific being the chasm left 
when the larger portion of the mass of the moon parted from the 
earth. 

Adopting, as I do, the theory of the origin of the earth by 
meteoric accretion of solid matter, we must consider our planet 
as having been produced from one of those vast rings of meteor- 
ites which in great numbers still circulate round the sun, but 
which at the much earlier period now contemplated were both 
more numerous and much more extensive. Owing to irregu- 
larities of distribution in such a ring and through disturbance 
by other bodies, aggregations of various sizes would inevitably 
occur, and the largest of these would in time draw in to itself 
all the rest, and thus form a planet. During the early stages 
of this process the particles would be so small, and would come 
together so gradually, that little heat would be produced, and 
there would result merely a loose aggregation of cold matter. 
But as the process went on and the mass of the incipient planet 
became considerable — perhaps half that of the earth — the rest 
of the ring would fall in with greater and greater velocity ; and 
this, added to the gravitative compression of the growing mass 
might, when nearly its present size, have produced sufficient heat 
to liquefy the outer layers, while the central portion remained 
solid and to some extent incoherent, with probably large quan- 
tities of heavy gases in the interstices. When the amount of the 
meteoric accretions became so reduced as to be insufficient to keep 



THE EARTH, IN RELATION TO LIFE 233 

up the heat to the melting-point a crust would form, and might 
have reached about half or three-fourths of its present thickness 
when the moon became separated. 

Let us now try to picture to ourselves what happened. We 
should have a globe somewhat larger than our earth is now, both 
because it then contained the material of the moon and also because 
it was hotter, revolving so rapidly as to be very greatly flattened 
at the poles ; while the equatorial belt bulged out enormously, and 
would probably have separated in the form of a ring with a 
very slight increase of the time of rotation, which is supposed 
to have been about four hours. This globe would have a com- 
paratively thin crust, beneath which there was molten rock to an 
unknown depth, perhaps a few hundreds, perhaps more than a 
thousand miles. At this time the attraction of the sun acting 
on the molten interior produced tides in it, causing the thin crust 
to rise and fall every two hours, but to so small an extent — 
only about a foot or so — as not necessarily to fracture it ; but it 
is calculated that this slight rhythmic undulation coincided with 
the normal period of undulation due to such a large mass of 
heavy liquid, and so tended to increase the instability due to 
rapid rotation. 

The bulk of the moon is about one-fiftieth part that of the 
earth, and an easy calculation shows us that, taking the area 
of the Pacific, Atlantic, and Indian Oceans combined as about 
two-thirds that of the globe, it would require a thickness (or 
depth) of about forty miles to furnish the material for the moon. 
We must, of course, assume that there were some inequalities in 
the thickness of the crust and in its comparative rigidity, so that 



234 MAN'S PLACE IN THE UNIVERSE 

when the critical moment came and the earth could no longer 
retain its equatorial protuberance against the centrifugal force 
due to rotation combined with the tidal undulations caused by 
the sun, instead of a continuous ring slowly detaching itself, 
the crust gave way in two or more great masses where it was 
weakest, and as the tidal wave passed under it and a quantity of 
the liquid substratum rose with it, the whole would break up and 
collect into a sub-globular mass a short distance from the earth, 
and continue revolving with it for some time at about the same 
rate as the surface had rotated. But as tidal action is always 
equal on opposite sides of a globe, there would be a similar dis- 
ruption there, forming, it may be supposed, the Atlantic basin, 
which, as may be seen on a small globe, is almost exactly opposite 
a part of the Central Pacific. So soon as these two great masses 
had separated from the earth, the latter would gradually settle 
down into a state of equilibrium, and the molten matter of the 
interior, which would now fill the great oceanic basins up to a 
level of a few miles below the general surface, would soon cool 
enough to form a thin crust. The larger portion of the nascent 
moon would gradually attract to itself the one or more smaller 
portions and form our satellite ; and from that time tidal friction 
by both moon and sun would begin to operate and would grad- 
ually lengthen our day and, more rapidly, our month in the way 
explained in Sir Robert Ball's volume. 

A very interesting point may now be referred to, because it 
seems confirmatory of this origin of the great ocean basins. In 
Mr. Osmond Fisher's work it is explained how the variations in 
the force of gravity, at numerous points all over the world, have 



THE EARTH, IN RELATION TO LIFE 235 

been determined by observations with the pendulum, and also how 
these variations afford a measure of the thickness of the solid 
crust, which is of less specific gravity than the molten interior 
on which it rests. By this means a very interesting result was ob- 
tained. The observations on numerous oceanic islands proved that 
the sub-oceanic crust was considerably more dense than the crust 
under the continents, but also thinner, the result being to bring the 
average mass of the sub-oceanic crust and oceans to an equality 
with that of the continental crust, and this causes the whirling 
earth to be in a state of balance, or equilibrium. Now, both the 
thinness and the increased density of the crust seem to be well ex- 
plained by this theory of the origin of the oceanic basins. The new 
crust would necessarily for a long time be thinner than the older 
portion, because formed so much later; but it would very soon 
become cool enough to allow the aqueous vapour of the atmos- 
sphere and that given off through fissures from the molten in- 
terior to collect in the ocean basins, which would thenceforth be 
cooled more rapidly and kept at a uniform temperature and also 
under a uniform pressure, and these conditions would lead to the 
steady and continuous increase of thickness, with a greater com- 
pactness of structure than in the continental areas. It is no 
doubt to this uniformity of conditions, with a lowering of the 
bottom temperature throughout the greater part of geological 
time, till it has become only a few degrees above the freezing- 
point, that we owe the remarkable persistence of the vast and 
deep ocean basins on which, as we have seen, the continuity of 
life on the earth has largely depended. 

There is one other fact which lends some support to this 



236 MAN'S PLACE IN THE UNIVERSE 

theory of the origin of the ocean basins — their almost complete 
symmetry with regard to the equator. Both the Atlantic and 
Pacific basins extend to an equal distance north and south of 
the equator, an equality which could hardly have been produced 
by any cause not directly connected with the earth's rotation. 
The polar seas which are co-terminous with the two great oceans 
are very much shallower, and cannot, therefore, be considered 
as forming part of the true oceanic basins. 

WATEE AS AN EQUALISER OF TEMPERATURE 

The importance of water in regulating the temperature of the 
earth is so great that, even if we had enough water on the land 
for all the wants of plants and animals, but had no great oceans, 
it is almost certain that the earth could not have produced and 
sustained the various forms of life which it now possesses. 

The effect of the oceans is twofold. Owing to the great spe- 
cific heat of water, that is, its property of absorbing heat slowly 
but to a large amount, and giving it out with equal slowness, the 
surface-waters of the oceans and seas are heated by the sun so 
that by the evening of a bright day they have become quite warm 
to a depth of several feet. But air has much less specific heat 
than water, a pound of water in cooling one degree being capable 
of warming four pounds of air one degree; but as air is 770 
times as light as water, it follows that the heat from one cubic 
foot of water will warm more than 3000 cubic feet of air as much 
as it cools itself. Hence the enormous surface of the seas and 
oceans, the larger part of which is within the tropics, warms the 



THE EARTH, IN RELATION TO LIFE 237 

whole of the lower and denser portions of the air, especially dur- 
ing the night, and this warmth is carried to all parts of the earth 
by the winds, and thus ameliorates the climate. Another quite 
distinct effect is due to the great ocean currents, like the Gulf 
Stream and the Japan Current, which carry the warm water of 
the tropics to temperate and arctic regions, and thus render 
many countries habitable which would otherwise suffer the rigour 
of an almost arctic winter. These currents are, however, directly 
due to the winds, and properly belong to the section on the 
atmosphere. 

The other equalising action, due primarily to the great area 
of the seas and oceans, is a result of the vast evaporating surface 
from which the land derives almost all its water in the form of 
rain and rivers ; and it is quite evident that if there were not suffi- 
cient water-surface to produce an ample supply of vapour for 
this purpose, arid districts would occupy more and more of the 
earth's surface. How much water-surface is necessary for life 
we do not know; but if the proportions of water and land-sur- 
faces were reversed, it seems probable that the larger proportion 
of the earth might be uninhabitable. The vapour thus produced 
has also a very great effect in equalising temperature ; but this 
also is a point which will come better under our next chapter on 
the atmosphere. 

There are, however, some matters connected with the water- 
supply of the earth, and its relation to the development of life, 
that call for a few remarks here. What has determined the 
total quantity of water on the earth or on other planets does 



238 MAN'S PLACE IN THE UNIVERSE 

not appear to be known ; but presumably it would depend, par- 
tially or wholly, on the mass of the planet being sufficient to 
enable it to retain by its gravitative force the oxygen and hydro- 
gen of which water is composed. As the two gases are so easily 
combined to form water, but can only be separated under special 
conditions, its quantity would be dependent on the supply of 
hydrogen, which is but rarely found on the earth in a free state. 
The important fact, however, is, that we do possess so great 
a quantity of water, that if the whole surface of the globe was 
as regularly contoured as are the continents, and merely wrinkled 
with mountain chains, then the existing water would cover the 
whole globe nearly two miles deep, leaving only the tops of high 
mountains above its surface as rows of small islands, with a few 
larger islands formed by what are now the high plateaus of 
Tibet and the Southern Andes. 

Now there seems no reason why this distribution of the water 
should not have occurred — in fact it seems probable that it would 
have occurred, had it not been for the fortunate coincidence of 
the formation of enormously deep ocean basins. So far as I 
am aware, no sufficient explanation of the formation of these 
basins has been given but that of Mr. Osmond Fisher, as here 
described, and that depends upon three unique circumstances: 
(1) the formation of a satellite at a very late period of the 
planet's development when there was already a rather thick 
crust; (£) the satellite being far larger in proportion to its 
primary than any other in the solar system ; and ( 3 ) its having 
been produced by fission from its primary on account of ex- 
tremely rapid rotation, combined with solar tides in its molten 



THE EARTH, IN RELATION TO LIFE 239 

interior, and a rate of oscillation of that molten interior coincid- 
ing with the tidal period. 1 

Whether this very remarkable theory of the origin of our 
moon is the true one, and if so, whether the explanation it seems 
to afford of the great oceanic basins is correct, I am not mathe- 
matician enough to judge. The tidal theory of the origin of the 
moon, as worked out mathematically by Professor G. H. Darwin, 
has been supported by Sir Robert Ball and accepted by many 
other astronomers; while the researches of the Rev. Osmond 
Fisher into the Physics of the Earth's Crust, together with his 
mathematical abilities and his practical work as a geologist, 
entitle his opinion on the question of the mode of origin of the 
ocean basins to the highest respect. And, as we have seen, the 
existence of these vast and deep ocean basins, produced by the 
agency of a series of events so remarkable as to be quite unique 
in the solar system, played an important part in rendering the 
earth fit for the development of the higher forms of animal life, 
while without them it seems not improbable that the conditions 
would have been such as to render any varied forms of terrestrial 
life hardly possible. 

1 Professor G. H. Darwin states that it is nearly certain that no other 
satellite nor any of the planets originated in the same way as the moon. 



CHAPTER XIII 

THE EARTH IN RELATION TO LIFE! ATMOSPHERIC CONDITIONS 

We have seen in our tenth chapter that the physical basis of life 
— protoplasm — consists of the four elements — oxygen, nitrogen, 
hydrogen, and carbon, and that both plants and animals depend 
largely upon the free oxygen in the air to carry on their vital 
processes ; while the carbonic acid and ammonia in the atmosphere 
seem to be absolutely essential to plants. Whether life could 
have arisen and have been highly developed with an atmosphere 
composed of different elements from ours it is, of course, impos- 
sible to say ; but there are certain physical conditions which seem 
absolutely essential whatever may be the elements which com- 
pose it. 

The first of these essentials is an atmosphere which shall be of 
such density at the surface of the planet, and of so great a bulk 
as to be not too rare to fulfil its various functions at all altitudes 
where there is a considerable area of land. What determines 
the total quantity of gaseous matter on the surface of a planet 
will be, mainly, its mass, together with the average temperature 
of its surface. 

The molecules of gases are in a state of rapid motion in all 
directions, and the lighter gases have the most rapid motions. 
The average speed of the motion of the molecules has been 

240 



THE AIR IN RELATION TO LIFE 241 

roughly determined under varying conditions of pressure and 
temperature, and also the probable maximum and minimum rates, 
and from these data, and certain known facts as to planetary 
atmospheres, Mr. G. Johnstone Stoney, F. R. S., has calculated 
what gases will escape from the atmospheres of the earth and 
the other planets. He finds that all the gases which are con- 
stituents of air have such comparatively low molecular rates of 
motion that the force of gravity at the upper limits of the earth's 
atmosphere is amply sufficient to retain them ; hence the stability 
of its composition. But there are two other gases, hydrogen 
and helium, which are both known to enter the atmosphere, but 
never accumulate so as to form any measurable portion of it, 
and these are found to have sufficient molecular motion to escape 
from it. With regard to hydrogen, if the earth were much 
larger and more massive than it is, so as to retain the hydrogen, 
disastrous consequences might ensue, because, whenever a suffi- 
cient quantity of this gas accumulated, it would form an ex- 
plosive mixture with the oxygen of the atmosphere, and a flash 
of lightning or even the smallest flame would lead to explosions 
so violent and destructive as perhaps to render such a planet un- 
suited for the development of life. We appear, therefore, to be 
just at the major limit of mass to secure habitability, except in 
such planets as may have no continuous supply of free hydrogen. 

Perhaps the most important mechanical functions of the at- 
mosphere dependent on its density are: (1) the production of 
winds, which in many ways bring about an equalisation of tem- 
perature, and which also produce surface-currents on the ocean ; 



242 MAN'S PLACE IN THE UNIVERSE 

and (£) the distribution of moisture over the earth by means 
of clouds which also have other important functions. 

Winds depend primarily on the local distribution of heat in 
the air, especially on the great amount of heat constantly present 
in the equatorial zone, due to the sun being always nearly vertical 
at noon, and to its being similarly vertical at each tropic once 
a year, with a longer day, leading to even higher temperatures 
than at the equator, and producing also that continuous belt of 
arid lands or deserts which almost encircle the globe in the region 
of the tropics. Heated air being lighter, the colder air from 
the temperate zones continually flows towards it, lifting it up 
and causing it to flow over, as it were, to the north and south. 
But as the inflow comes from an area of less rapid to one of 
more rapid rotation, the course of the air is diverted, and pro- 
duces the northeast and southeast trades ; while the overflow from 
the equator going to an area of less rapid rotation turns west- 
ward and produces the southwest winds so prevalent over the 
north Atlantic and north temperate zone generally, and the 
northwest in the southern hemisphere. 

It is outside the zone of the equable trade-winds, and in a 
region a few degrees on each side of the tropics, that destructive 
hurricanes and typhoons prevail. These are really enormous 
whirlwinds due to the intensely heated atmosphere over the arid 
regions already mentioned, causing an inrush of cool air from 
various directions, thus setting up a rotatory motion which in- 
creases in rapidity till equilibrium is restored. The hurricanes 
of the West Indies and Mauritius, and the typhoons of the 
Eastern seas, are thus caused. Some of these storms are so 



THE AIR IN RELATION TO LIFE 243 

violent that no human structures can resist them, while the largest 
and most vigorous trees are torn to pieces or overturned by them. 
But if our atmosphere were much denser than it is, its increased 
weight would give it still greater destructive force; and if to 
this were added a somewhat greater amount of sun-heat — which 
might be due either to our greater proximity to the sun or to 
the sun's greater size or greater heat-intensity, these tempests 
might be so increased in violence and frequency as to render con- 
siderable portions of the earth uninhabitable. 

The constant and equable trade-winds have a very important 
function in initiating those far-reaching ocean-currents which 
are of the greatest importance in equalising temperature. The 
well-known Gulf Stream is to us the most important of these cur- 
rents, because it plays the chief part in giving us the mild climate 
we enjoy in common with the whole of Western Europe, a mild- 
ness which is felt to a considerable distance within the Arctic 
Circle; and, in conjunction with the Japan Current, which does 
the same for the whole of the temperate regions of the North 
Pacific, renders a large portion of the globe better adapted for 
life than it would be without these beneficial influences. 

These equalising currents, however, are almost entirely due 
to the form and position of the continents, and especially to the 
fact that they are so situated as to leave vast expanses of ocean 
along the equatorial zone, and extending north and south to the 
Arctic and Antarctic regions. If with the same amount of land 
the continents had been so grouped as to occupy a considerable 
portion of the equatorial oceans — such as would have been the 
case had Africa been turned so as to join South America, and 



244 MAN'S PLACE IN THE UNIVERSE 

Asia been brought to the southeast so as to take the place of 
part of the equatorial Pacific, then the great ocean-currents 
would have been but feeble or have hardly existed. Without 
these currents much of the north and south temperate lands 
would have been buried in ice, while the largest portion of the 
continents would have been so intensely heated as perhaps to be 
unsuited for the development of the higher forms of animal life, 
since we have shown (in Chapters X and XI) how delicate is the 
balance and how narrow the limits of temperature which are 
required. 

There seems to be no reason whatever why some such distribu- 
tion of the sea and land should not have existed, had it not been 
for the admittedly exceptional conditions which led to the pro- 
duction of our satellite, thus necessarily forming vast chasms 
along the region of the equator where centrifugal force as well 
as the internal solar tides were most powerful, and where the thin 
crust was thus compelled to give way. And as the highest 
authorities declare that there are no indications of such an origin 
of satellites in the case of any other planet, the whole series of 
conditions favourable to life on the earth become all the more 
remarkable. 

CLOUDS ; THEIR IMPORTANCE AND THEIR CAUSES 

Few persons have any adequate conception of the real nature 
of clouds and of the important part they take in rendering our 
world a habitable and an enjoyable one. 

On the average, the rainfall over the oceans is much less than 
over the land, the whole region of the trade-winds having usually 



THE AIR IN RELATION TO LIFE 245 

a cloudless sky and very little rain; but in the intervening belt 
of calms, near to the equator, a cloudy sky and heavy rains are 
frequent. This arises from the fact that the warm, moist air 
over the ocean is raised upwards, by the cold and heavy air from 
north and south, into a cooler region where it cannot hold so much 
aqueous vapour, which is there condensed and falls as rain. Gen- 
erally, wherever the winds blow over extensive areas of water on 
to the land, especially if there are mountains or elevated plateaus 
which cause the moisture-laden air to rise to heights where the 
temperature is lower, clouds are formed and more or less rain 
falls. But if the land is of an arid nature and much heated by 
the sun, the air becomes capable of holding still more aqueous 
vapour, and even dense rain-clouds disperse without producing 
any rainfall. From these simple causes, with the large area 
of sea as compared with the land upon our earth, by far the 
larger portion of the surface is well supplied with rain, which, 
falling most abundantly in the elevated and therefore cooler 
regions, percolates the soil, and gives rise to those innumerable 
springs and rivulets which moisten and beautify the earth, and 
which, uniting together, form streams and rivers, which return 
to the seas and oceans whence they were originally derived. 

CLOUDS AND RAIN DEPEND UPON ATMOSPHERIC DUST 

The beautiful system of aqueous circulation by means of the 
atmosphere as sketched above was long thought to explain the 
whole process, and to require no further elucidation ; but about 
a quarter of a century back a curious experiment was made which 



246 MAN'S PLACE IN THE UNIVERSE 

indicated that there was another factor in the process which had 
been entirely overlooked. If a small jet of steam is sent into two 
large glass receivers, one filled with ordinary air, the other with 
air which has been filtered by passing through a thick layer of 
cotton wool so as to keep back all particles of solid matter, the 
first vessel will be instantly filled with condensed, cloudy-looking 
vapour, while in the other vessel the air and vapour will remain 
perfectly transparent and invisible. Another experiment was 
then made to imitate more nearly what occurs in nature. The 
two vessels were prepared as before, but a small quantity of 
water was placed in each vessel and allowed to evaporate till the 
air was nearly saturated with vapour, which remained invisible 
in both. Both vessels were then slightly cooled, when instantly 
a dense cloud was formed in that filled with unfiltered air, while 
the other remained quite clear. These experiments proved that 
the mere cooling of air below the dew point will not cause the 
aqueous vapour in it to condense into drops so as to form mist, 
fog, or cloud, unless small particles of solid or liquid matter are 
present to act as nuclei upon which condensation begins. The 
density of a cloud will therefore depend not only on the quantity 
of vapour in the air, but on the presence of an abundance of 
minute dust-particles on which condensation can begin. 

That such dust exists everywhere in the air, even up to great 
heights, is not a supposition but a proved fact. By exposing 
glass plates covered with glycerine in different places and at dif- 
ferent altitudes the number of these particles in each cubic foot 
of air has been determined ; and it is found that not only are they 
present everywhere at low levels, but that there are a considerable 



THE AIR IN RELATION TO LIFE 247 

number even at the tops of the highest mountains. These solid 
particles also act in another way. By radiation in the higher 
atmosphere they become very cold, and thus condense the vapour 
by contact, just as the points of grass-blades condense it to form 
dew. 

When steam is escaping from an engine we see a mass of dense 
white vapour, a miniature cloud; and if we are near it in cold, 
damp weather, we feel little drops of rain produced from it. But 
on a fine, warm day it rises quickly and soon melts away, and 
entirely disappears. Exactly the same thing happens on a 
larger scale in nature. In fine weather we may have abundant 
clouds continually passing high overhead, but they never produce 
rain, because as the minute globules of water slowly fall towards 
the earth, the warm, dry air again turns them into invisible 
vapour. Again, in fine weather, we often see a small cloud on 
a mountain top which remains there a considerable time, even 
though a brisk wind is blowing. The mountain top is colder 
than the surrounding air, and the invisible vapour becomes con- 
densed into cloud by passing over it, but the moment these cloud 
particles are carried past the summit into the warmer and drier 
air they are again evaporated and disappear. On Table Moun- 
tain, near Cape Town, this phenomenon occurs on a large scale, 
and is termed the table-cloth, the mass of white fleecy cloud seem- 
ing to hang over the flat mountain top to some distance down, 
where it remains for several months, while all around there is 
bright sunshine. 

Another phenomenon that indicates the universal presence of 
dust to enormous heights in the atmosphere is the blue colour of 



248 MAN'S PLACE IN THE UNIVERSE 

the sky. This is caused by the presence of such excessively 
minute particles of dust through an enormous thickness of the 
higher atmosphere — probably up to a height of twenty or thirty 
miles, or more — that they reflect only the light of short wave- 
lengths from the blue end of the spectrum. This also has been 
proved by experiment. If a glass cylinder, several feet long, 
is filled with pure air from which all solid particles have been 
removed by filtering and passing over red-hot platinum wires, 
and a ray of electric light is passed through it, the interior, when 
viewed laterally, appears quite dark, the light passing through 
in a straight line and not illuminating the air. But if a little 
more air is passed through the filter, but so rapidly as to allow 
the minutest particles of dust to enter with it, the vessel becomes 
gradually filled with a blue haze, which gradually deepens into 
a beautiful blue, comparable with that of the sky. If now some 
of the unfiltered air is admitted, the blue fades away into the 
ordinary tint of daylight. 

Since it has been known that liquid oxygen is blue, many people 
have concluded that this explains the blue colour of the sky. But 
it has really nothing to do with the point at issue. The blue 
of the liquid oxygen becomes so excessively faint in the gas, 
further attenuated as it is by the colourless nitrogen, that it 
would have no perceptible colour in the whole thickness of our 
atmosphere. Again, if it had a perceptible blue tint we could 
not see it against the blackness of space behind it; but white 
objects seen through it, such as the moon and clouds, should all 
appear blue, which they do not do. The blue we see is from the 
whole sky, and is therefore reflected light; and as pure air is 



THE AIR IN RELATION TO LIFE 249 

quite transparent, there must be solid or liquid particles so 
minute as to reflect blue light only. In the lower atmosphere 
the rain-producing particles are larger, and reflect all the rays, 
thus diluting the blue colour near the horizon, and, by refraction 
and reflection combined, producing the various beautiful hues of 
sunrise and sunset. 

This production of exquisite colours by the dust in the at- 
mosphere, though adding greatly to the enjoyment of life, can- 
not be considered essential to it ; but there is another circumstance 
connected with atmospheric dust which, though little appreciated, 
might have effects which can hardly be calculated. If there 
were no dust in the atmosphere, the sky would appear black even 
at noon, except in the actual direction of the sun ; and the stars 
would be visible in the day as well as at night. This would follow 
because air does not reflect light, and is not visible. We should 
therefore receive no light from the sky itself as we do now, and 
the north side of every hill, house, and other solid objects, would 
be totally dark, unless there were any surfaces in that direction 
to reflect the light. The surface of the ground at a little dis- 
tance would be in sunshine, and this would be the only source of 
light wherever direct sunlight was cut off. To get a good 
amount of pleasant light in houses it would be necessary to have 
them built on nearly level ground, or on ground rising to the 
north, and with walls of glass all round and down to the floor 
line, to receive as much as possible of the reflected light from the 
ground. What effect this kind of light would have on vegeta- 
tion it is difficult to say, but trees and shrubs would probably 
grow laterally towards the south, east, and west, so as to get 



260 MAN'S PLACE IN THE UNIVERSE 

as much direct sunshine as possible. A more important result 
would be that, as sunshine would be perpetual during the day, 
so much evaporation would take place that the soil would become 
arid and almost bare in places that are now covered with vegeta- 
tion, and plants like the cactuses of Arizona and the euphor- 
bias of South Africa would occupy a large portion of the 
surface. 

Returning now from this collateral subject of light and colour 
to the more important aspect of the question — the absence of 
cloud and rain — we have to consider what would happen, and in 
what way the enormous quantity of water which would be 
evaporated under continual sunshine would be returned to 
the earth. 

The first and most obvious means would be by abnormally 
abundant dews, which would be deposited almost every night on 
every form of leafy vegetation. Not only would all grass and 
herbage, but all the outer leaves of shrubs and trees, condense so 
much moisture as to take the place of rain so far as the needs of 
such vegetation were concerned. But without arrangements for 
irrigation cultivation would be almost impossible, because the 
bare soil would become intensely heated during the day, and would 
retain so much of its heat through the night as to prevent any 
dew forming upon it. 

Some more effective mode, therefore, of returning the aqueous 
vapour of the atmosphere to the earth and ocean, would be re- 
quired, and this, I believe, would be done by means of hills and 
mountains of sufficient height to become decidedly colder than 
the lowlands. The air from over the oceans would be constantly 



THE AIR IN RELATION TO LIFE 2dl 

loaded with moisture, and whenever the winds blew on to the land 
the air would be carried up the slopes of the hills into the colder 
regions, and there be rapidly condensed upon the vegetation, and 
also on the bare earth and rocks of northern slopes, and wherever 
they cooled sufficiently during the afternoon or night to be below 
the temperature of the air. The quantity of vapour thus con- 
densed would reduce the atmospheric pressure, which would lead 
to an inrush of air from below, bringing with it more vapour, 
and this might give rise to perpetual torrents, especially on 
northern and eastern slopes. But as the evaporation would be 
much greater than at the present time, owing to perpetual sun- 
shine, so the water returned to the earth would be greater, and as 
it would not be so uniformly distributed over the land as it is now, 
the result would perhaps be that extensive mountain sides would 
become devastated by violent torrents, rendering permanent 
vegetation almost impossible; while other and more extensive 
areas, in the absence of rain, would become arid wastes that would 
support only the few peculiar types of vegetation that are char- 
acteristic of such regions. 

Whether such conditions as here supposed would prevent the 
development of the higher forms of life it is impossible to say, 
but it is certain that they would be very unfavourable, and might 
have much more disastrous consequences than any we have here 
suggested. We can hardly suppose that, with winds and rock- 
formations at all like what they are now, any world could be 
wholly free from atmospheric dust. If, however, the atmosphere 
itself were much less dense than it is, say one-half, which might 
very easily have been the case, then the winds would have less 



252 MAN'S PLACE IN THE UNIVERSE 

carrying power, and at the elevations at which clouds are usually 
formed there would not be enough dust-particles to assist in their 
formation. Hence fogs close to the earth's surface would largely 
take the place of clouds floating far above it, and these would 
certainly be less favourable to human life and to that of many of 
the higher animals than existing conditions. 

The world-wide distribution of atmospheric dust is a remark- 
able phenomenon. As the blue colour of the sky is universal, the 
whole of the higher atmosphere must be pervaded by myriads of 
ultra-microscopical particles, which by reflecting the blue rays 
only give us not only the azure vault of heaven, but in combina- 
tion with the coarser dust of lower altitudes, diffused daylight, 
the grand forms and motions of the fleecy clouds, and the " gentle 
rain from heaven " to refresh the parched earth and make it 
beautiful with foliage and flowers. Over every part of the vast 
Pacific Ocean, whose islands must produce a minimum of dust, 
the sky is always blue, and its thousand isles do not suffer for 
want of rain. Over the great forest-plain of the Amazon valley, 
where the production of dust must be very small, there is yet 
abundance of rain-clouds and of rain. This is due primarily 
to the two great natural sources of dust — the active volcanoes, 
together with the deserts and more arid regions of the world; 
and, in the second place, to the density and wonderful mobility 
of the atmosphere, which not only carries the finest dust-particles 
to an enormous height, but distributes them through its whole 
extent with such wonderful uniformity. 

Every dust particle is of course much heavier than air, and in 
a comparatively short time, if the atmosphere were still, would 



THE AIR IN RELATION TO LIFE 253 

fall to the ground. Tyndall found that the air of a cellar under 
the Royal Institution in Albemarle Street, which had not been 
opened for several months, was so pure that the path of a beam 
of electric light sent through it was quite invisible. But careful 
experiments show that not only is the air in continual motion, but 
the motion is excessively irregular, being hardly ever quite hori- 
zontal, but upwards and downwards and in every intermediate 
direction, as well as . in countless whirls and eddies ; and this 
complexity of motion must extend to a vast height, probably 
to fifty miles or more, in order to provide a sufficient thick- 
ness of those minutest particles which produce the blue of the 
sky. 

All this complexity of motion is due to the action of the sun 
in heating the surface of the earth, and the extreme irregularity 
of that surface both as regards contour and its capacity for 
heat-absorption. In one area we have sand or rock or bare clay, 
which, when exposed to bright sunshine, becomes scorching hot ; 
in another area we have dense vegetation, which, owing to evap- 
oration caused by the sunshine, remains comparatively cool, and 
also the still cooler surfaces of rivers and Alpine lakes. But if 
the air were much less dense than it is, these movements would 
be less energetic, while all the dust that was raised to any con- 
siderable height would, by its own weight, fall back again to the 
earth much more rapidly than it does now. There would thus be 
much less dust permanently in the atmosphere, and this would 
inevitably lead to diminished rainfall and, partially, to the other 
injurious effects already described. 



254 MAN'S PLACE IN THE UNIVERSE 

ATMOSPHERIC ELECTRICITY 

We have already seen that vegetable organisms obtain the chief 
part of the nitrogen in their tissues from ammonia produced in 
the atmosphere and carried into the earth by rain. This sub- 
stance can only be thus produced by the agency of electrical dis- 
charges, or lightning, which cause the combination of the hydro- 
gen in the aqueous vapour with the free oxygen of the air. But 
clouds are important agents in the accumulation of electricity in 
sufficient amount to produce the violent discharges we know as 
lightning, and it is doubtful whether without them there would 
be any discharges through the atmosphere capable of decompos- 
ing the aqueous vapour in it. Not only are clouds beneficial in 
the production of rain, and also in moderating the intensity of 
continuous sun-heat, but they are also requisite for the formation 
of chemical compounds in vegetables which are of the highest 
importance to the whole animal kingdom. So far as we know, 
animal life could not exist on the earth's surface without this 
source of nitrogen, and therefore without clouds and lightning ; 
and these, we have just seen, depend primarily on a due propor- 
tion of dust in the atmosphere. 

But this due proportion of dust is mainly supplied by volcanoes 
and deserts, and its distribution and constant presence in the 
air depend upon the density of the atmosphere. This again 
depends on two other factors: the force of gravity due to the 
mass of the planet, and the absolute quantity of the free gases 
constituting the atmosphere. 

We thus find that the vast, invisible ocean of air in which we 



THE AIR IN RELATION TO LIFE 255 

live, and which is so important to us that deprivation of it for 
a few minutes is destructive of life, produces also many other 
beneficial effects of which we usually take little account, except 
at times when storm or tempest, or excessive heat or cold, remind 
us how delicate is the balance of conditions on which our com- 
fort, and even our lives, depend. 

But the sketch I have here attempted to give of its varied 
functions shows us that it is really a most complex structure, a 
wonderful piece of machinery, as it were, which in its various 
component gases, its actions and reactions upon the water and 
the land, its production of electrical discharges, and its furnish- 
ing the elements from which the whole fabric of organic life is 
composed and perpetually renewed, may be truly considered to be 
the very source and foundation of life itself. This is seen, not 
only in the fact of our absolute dependence upon it every minute 
of our lives, but in the terrible effects produced by even a slight 
degree of impurity in this vital element. Yet it is among those 
nations that claim to be the most civilised, those that profess to 
be guided by a knowledge of the laws of nature, those that most 
glory in the advance of science, that we find the greatest apathy, 
the greatest recklessness, in continually rendering impure this 
all-important necessary of life, to such a degree that the health 
of the larger portion of their populations is injured and their 
vitality lowered, by conditions which compel them to breathe more 
or less foul and impure air for the greater part of their lives. 
The huge and ever-increasing cities, the vast manufacturing 
towns belching forth smoke and poisonous gases, with the 
crowded dwellings, where millions are forced to live under the 



256 MAN'S PLACE IN THE UNIVERSE 

most terrible insanitary conditions, are the witnesses to this 

criminal apathy, this incredible recklessness and inhumanity. 

For the last fifty years and more the inevitable results of such 
conditions have been fully known; yet to this day nothing of 
importance has been done, nothing is being done. In this beau- 
tiful land there is ample space and a superabundance of pure air 
for every individual. Yet our wealthy and our learned classes, 
our rulers and law-makers, our religious teachers and our men 
of science, all alike devote their lives and energies to anything or 
everything but this. Yet this is the one great and primary 
essential of a people's health and well-being, to which everything 
should, for the time, be subordinate. Till this is done, and done 
thoroughly and completely, our civilisation is naught, our science 
is naught, our religion is naught, and our politics are less than 
naught — are utterly despicable; are below contempt. 

It has been the consideration of our wonderful atmosphere in 
its various relations to human life, and to all life, which has com- 
pelled me to this cry for the children and for outraged humanity. 
Will no body of humane men and women band themselves together, 
and take no rest till this crying evil is abolished, and with it 
nine-tenths of all the other evils that now afflict us? Let every- 
thing give way to this. As in a war of conquest or aggression 
nothing is allowed to stand in the way of victory, and all private 
rights are subordinated to the alleged public weal, so, in this 
war against filth, disease, and misery let nothing stand in the 
way — neither private interests nor vested rights — and we shall 
certainly conquer. This is the gospel that should be preached, 
in season and out of season, till the nation listens and is convinced. 



THE AIR IN RELATION TO LIFE 257 

Let this be our claim : Pure air and pure water for every inhabi- 
tant of the British Isles. Vote for no one who says " It can't 
be done." Vote only for those who declare " It shall be done." 
It may take five or ten or twenty years, but all petty ameliora- 
tions, all piecemeal reforms, must wait till this fundamental 
reform is effected. Then, when we have enabled our people to 
breathe pure air, and drink pure water, and live upon simple 
food, and work and play and rest under healthy conditions, they 
will be in a position to decide (for the first time) what other 
reforms are really needed. 

Remember ! We claim to be a people of high civilisation, of 
advanced science, of great humanity, of enormous wealth ! For 
very shame do not let us say " We cannot arrange matters so that 
our people may all breathe unpolluted, unpoisoned air ! " 



CHAPTER XIV 

THE EARTH IS THE ONLY HABITABLE PLANET IN 
THE SOLAR SYSTEM 

Having shown in the last three chapters how numerous and how 
complex are the conditions which alone render life possible on 
our earth, how nicely balanced are opposing forces, and how 
curious and delicate are the means by which the essential combina- 
tions of the elements are brought about, it will be a comparatively 
easy task to show how totally unfitted are all the other planets 
either to develop or to preserve the higher forms of life, and, in 
most cases, any forms above the lowest and most rudimentary. 
In order to make this clear we will take the most important of the 
conditions in order, and see how the various planets fulfil them. 

MASS OF A PLANET AND ITS ATMOSPHERE 

The height and density of the atmosphere of a planet is im- 
portant as regards life in several ways. On its density depends 
its power of carrying moisture ; of holding a sufficient supply of 
dust-particles for the formation of clouds; of carrying ultra- 
microscopic particles to such a height and in such quantity as 
to diffuse the light of the sun by reflection from the whole sky ; 
of raising waves in the ocean and thus aerating its waters, and 

258 



THE ONLY HABITABLE PLANET 259 

of producing the ocean currents which so greatly equalise tem- 
perature. Now this density depends on two factors: the mass 
of the planet and the quantity of the atmospheric gases. But 
there is good reason to think that the latter depends directly 
upon the former, because it is only when a certain mass is at- 
tained that any of the lighter permanent gases can be held on 
the surface of a planet. Thus, according to Dr. G. Johnstone 
Stoney, who has specially studied this subject, the moon cannot 
retain even such a heavy gas as carbonic acid, or the still heavier 
carbon disulphide; while no particle of oxygen, nitrogen, or 
water-vapour can possibly remain on it, owing to the fact of its 
mass being only about one eightieth that of the earth. It is 
believed that there are considerable quantities of gases in the 
stellar spaces, and probably also within the solar system, but 
perhaps in the liquid or solid form. In that state they might 
be attracted by any small mass such as the moon, but the heat 
of its surface when exposed to the solar rays would quickly re- 
store them to the gaseous condition, when they would at once 
escape. 

It is only when a planet attains a mass at least a quarter that 
of the earth that it is capable of retaining water-vapour, one 
of the most essential of the gases ; but with so small a mass as this 
its whole atmosphere would probably be so limited in amount 
and so rare at the planet's surface that it would be quite unable 
to fulfil the various purposes for which an atmosphere is required 
in order to support life. For their adequate fulfilment the mass 
of a planet cannot be much less than that of the earth. Here 
we come to one of those nice adjustments of which so many have 



260 MAN'S PLACE IN THE UNIVERSE 

been already pointed out. Dr. Johnstone Stoney arrives at the 
conclusion that hydrogen escapes from the earth. It is contin- 
ually produced in small quantities by submarine volcanoes, by 
fissures in volcanic regions, from decaying vegetation, and 
from some other sources; yet, though sometimes found in 
minute quantities, it forms no regular constituent of our 
atmosphere. 1 

The quantity of hydrogen combined with oxygen to form the 
mass of water in our vast and deep oceans is enormous. Yet if 
it had been only one-tenth more than it actually is the present 
land surface would have been almost all submerged. How the 
adjustments occurred so that there was exactly enough hydrogen 
to fill the vast ocean basins with water to such a depth as to leave 
enough land surface for the ample development of vegetable and 
animal life, and yet not so much as to be injurious to climate, it 
is difficult to imagine. Yet the adjustment stares us in the face. 
First we have a satellite unique in size as compared with its 
primary, and apparently in lateness of origin; then we have a 
mode of origin for that satellite said to be certainly unique in 
the solar system; as a consequence of this origin, it is believed, 
we have enormously deep ocean basins symmetrically placed with 
regard to the equator — an arrangement which is very important 
for ocean circulation ; then we must have had the right quantity 
of hydrogen, obtained in some unknown way, which formed water 
enough to fill these chasms, so as to leave an ample area of dry 

1 Transactions of Royal Dublin Society, vol. vi. (ser. ii.), part xiii. "Of 
Atmospheres upon Planets and Satellites." By G. Johnstone Stoney, F. R. 
S„ etc., etc. 



THE ONLY HABITABLE PLANET 261 

land, but which one-tenth more water would have ingulfed ; and, 
lastly, we have oxygen enough left to form an atmosphere of 
sufficient density for all the requirements of life. It could not 
be that the surplus hydrogen escaped when the water had been 
produced, because it escapes very slowly, and it combines so easily 
with free oxygen by means of even a spark, as to make it certain 
that all the available hydrogen was used up in the oceanic waters, 
and that the supply from the earth's interior has been since com- 
paratively small in amount. 

There is yet one more adjustment to be noticed. All the facts 
now referred to show that the earth's mass is sufficient to bring 
about the conditions favourable for life. But if our globe had 
been a little larger, and proportionately denser, in all probability 
no life would have been possible. Between a planet of 8000 and 
one of 9500 miles diameter is not a large difference, when com- 
pared with the enormous range of size of the other planets. Yet 
this slight increase in diameter would give two-thirds increase 
in bulk, and, with a corresponding increase of density due to the 
greater gravitative force, the mass would be about double what 
it is. But with double the mass the quantity of gases of all sorts 
attracted and retained by gravity would probably have been 
double ; and in that case there would have been double the quan- 
tity of water produced, as no hydrogen could then escape. But 
the surface of the globe would only be one-half greater than at 
present, in which case the water would have sufficed to cover the 
whole surface several miles deep. 



262 MAN'S PLACE IN THE UNIVERSE 

HABITABILITY OF OTHER PLANETS 

When we look to the other planets of our system we see every- 
where illustrations of the relation of size and mass to habitability. 
The smaller planets, Mercury and Mars, have not sufficient 
mass to retain water-vapour, and without it they cannot be 
habitable. All the larger planets can have very little solid 
matter, as indicated by their very low density, notwithstanding 
their enormous mass. There is, therefore, very good reason for 
the belief that the adaptability of a planet for a full development 
of life is 'primarily dependent, within very narrow limits, on its 
size and, more directly, on its mass. But if the earth owes its 
specially constituted atmosphere and its nicely adjusted quantity 
of water to such general causes as here indicated, and the same 
causes apply to the other planets of the solar system, then the 
only planet on which life can be possible is Venus. As, however, 
it may be urged that exceptional causes may have given other 
planets an equal advantage in the matter of air and water, we 
will briefly consider some of the other conditions which we have 
found to be essential in the case of the earth, but which it is 
almost impossible to conceive as existing, to the required extent, 
on any other planet of the solar system. 

A SMALL AND DEFINITE RANGE OF TEMPERATURE 

We have already seen within what narrow limits the temperature 
on a planet's surface must be maintained in order to develop and 
support life. We have also seen how numerous and how delicate 



THE ONLY HABITABLE PLANET 263 

are the conditions, such as density of atmosphere, extent and 
permanence of oceans, and distribution of sea and land, 
which are requisite, even with us, in order to render possible 
the continuous preservation of a sufficiently uniform tempera- 
ture. Slight alterations one way or another might render the 
earth almost uninhabitable, through its being liable to alterna- 
tions of too great heat or excessive cold. How then can we 
suppose that any other of the planets, which have either very 
much more or very much less sun-heat than we receive, 
could, by any possible modification of conditions, be rendered 
capable of producing and supporting a full and varied life- 
development? 

Mars receives less than half the amount of sun-heat per unit 
of surface that we do. And as it is almost certain that it con- 
tains no water (its polar snows being caused by carbonic acid 
or some other heavy gas) it follows that, although it may pro- 
duce vegetable life of some low kinds, it must be quite unsuited 
for that of the higher animals. Its small size and mass, the 
latter only one-ninth that of the earth, may probably allow it 
to possess a very rare atmosphere of oxygen and nitrogen, if 
those gases exist there, and this lack of density would render it 
unable to retain during the night the very moderate amount of 
heat it might absorb during the day. This conclusion is sup- 
ported by its low reflecting power, showing that it has hardly 
any clouds in its scanty atmosphere. During the greater part 
of the twenty-four hours, therefore, its surface-temperature 
would probably be much below the freezing point of water ; and 
this, taken in conjunction with the total absence of aqueous 



264 MAN'S PLACE IN THE UNIVERSE 

vapour or liquid water, would add still further to its unsuit- 

ability for animal life. 

In Venus the conditions are equally adverse in the other direc- 
tion. It receives from the sun almost double the amount of heat 
that we receive, and this alone would render necessary some ex- 
traordinary combination of modifying agencies in order to 
reduce and render uniform the excessively high temperature. 
But it is now known that Venus has one peculiarity which is in 
itself almost prohibitive of animal life, and probably of even the 
lowest forms of vegetable life. This peculiarity is, that through 
tidal action caused by the sun, its day has been made to coincide 
with its year, or, more properly, that it rotates on its axis in 
the same time that it revolves round the sun. Hence it always 
presents the same face to the sun ; and while one-half has a per- 
petual day, the other half has perpetual night, with perpetual 
twilight through refraction in a narrow belt adjoining the illu- 
minated half. But the side that never receives the direct rays of 
the sun must be intensely cold, approximating, in the central 
portions, to the zero of temperature, while the half exposed to 
perpetual sunshine of double intensity to ours, must almost cer- 
tainly rise to a temperature far too great for the existence of 
protoplasm, and probably, therefore, of any form of animal 
life. 

Venus appears to have a dense atmosphere, and its brilliancy 
suggests that we see the upper surface of a cloud-canopy, and 
this would no doubt greatly reduce the excessive solar heat. Its 
mass, being a little more than three-fourths that of the earth, 
would enable it to retain the same gases as we possess. But 



THE ONLY HABITABLE PLANET 265 

under the extraordinary conditions that prevail on the surface 
of this planet, it is hardly possible that the temperature of the 
illuminated side can be preserved in a sufficient state of uni- 
formity for the development of life in any of its higher 
forms. 

Mercury possesses the same peculiarity of keeping one face 
always toward the sun, and as it is so much smaller and so much 
nearer the sun its contrasts of heat and cold must be still more 
excessive, and we need hardly discuss the possibility of this planet 
being habitable. Its mass being only one-thirtieth that of the 
earth, water-vapour will certainly escape from it, and, most prob- 
ably, nitrogen and oxygen also, so that it can possess very little 
atmosphere; and this is indicated by its low reflecting power, no 
less than 83 per cent, of the sun's light being absorbed, and only 
17 per cent, reflected, whereas clouds reflect 72 per cent. This 
planet is therefore intensely heated on one side and frozen on the 
other ; it has no water and hardly any atmosphere, and is there- 
fore, from every point of view, totally unfitted for supporting 
living organisms. 

Even if it is supposed that, in the case of Venus, its per- 
petual cloud-canopy may keep down the surface temperature 
within the limits necessary for animal life, the extraordinary 
turmoil in its atmosphere caused by the excessively contrasted 
temperatures of its dark and light hemispheres must be ex- 
tremely inimical to life, if not absolutely prohibitive of it. For 
on the greater part of the hemisphere that never receives a ray of 
light or heat from the sun all the water and aqueous vapour must 
be turned into ice or snow, and it seems almost impossible that 



266 MAN'S PLACE IN THE UNIVERSE 

the air itself can escape congelation. It could only do so by a 
very rapid circulation of the whole atmosphere, and this would 
certainly be produced by the enormous and permanent difference 
of temperature between the two hemispheres. Indications of 
refraction by a dense atmosphere are visible during the planet's 
transit over the sun's disc, and also when it is in conjunction with 
the sun, and the refraction is so great that Venus is believed to 
have an atmosphere much higher than ours. But during the 
rapid circulation of such an atmosphere heated on one-half the 
planet and cooled on the other, most of the aqueous vapour must 
be taken out of it on the dark side as fast as it is produced on 
the heated side, though sufficient may remain to produce a canopy 
of very lofty clouds analogous to our cirri. The occasional 
visibility of the dark side of Venus may be caused by an elec- 
trical glow due to the friction of the perpetually overflowing and 
inflowing atmosphere, this being increased by reflection from a 
vast surface of perpetual snow. If we consider all the excep- 
tional features of this planet, it appears certain that the con- 
ditions as regards climate cannot now be such as to maintain a 
temperature within the narrow limits essential for life, while 
there is little probability that at any earlier period it can have 
possessed and maintained the necessary stability during the long 
epochs which are requisite for its development. 

Before considering the condition of the larger planets, it will 
be well to refer to an argument which has been supposed to 
minimise the difficulties already stated as to those planets which 
approach nearest to the earth in size and distance from the 
sun. 



THE ONLY HABITABLE PLANET 267 

THE ARGUMENT FROM EXTREME CONDITIONS ON THE EARTH 

In reply to the evidence showing how nice are the adaptations 
required for life-development, it is often objected that life does 
now exist under very extreme conditions — under tropic heat and 
arctic snows ; in the burnt-up desert as well as in the moist tropi- 
cal forest; in the air. as well as in the water; on lofty mountains 
as well as on the level lowlands. This is no doubt true, but it 
does not prove that life could have been developed in a world 
where any of these extremes of climate characterised the whole 
surface. The deserts are inhabited because there are oases 
where water is attainable, as well as in the surrounding fertile 
areas. The arctic regions are inhabited because there is a sum- 
mer, and during that summer there is vegetation. If the surface 
of the ground were always frozen, there would be no vegetation 
and no animal life. 

The late Mr. R. A. Proctor put this argument of the diversity 
of conditions under which life actually does exist on the earth 
as well probably as it can be put. He says : " When we consider 
the various conditions under which life is found to prevail, that 
no difference of climatic relations, or of elevation, of land, or 
of air, or of water, of soil in land, of freshness or saltness in 
water, of density in air, appears (so far as our researches have 
extended) to render life impossible, we are compelled to infer that 
the power of supporting life is a quality which has an exceed- 
ingly wide range in nature." 

This is true, but with certain reservations. The only species of 
animal which does really exist under the most varied conditions of 



268 MAN'S PLACE IN THE UNIVERSE 

climate is man, and he does so because his intellect renders him to 
some extent the ruler of nature. None of the lower animals 
have such a wide range, and the diversity of conditions is not 
really so great as it appears to be. The strict limits are 
nowhere permanently overpassed, and there is always the 
change from winter to summer, and the possibility of migra- 
tion to less inhospitable areas. 

THE GREAT PLANETS ALL UNINHABITABLE 

Having already shown that the condition of Mars, both as 
regards water, atmosphere, and temperature, is quite unfitted 
to maintain life, a view in which both general principles and tele- 
scopic examination perfectly agree, we may pass on to the outer 
planets, which, however, have long been given up as adapted for 
life even by the most ardent advocates for " life in other worlds." 
Their remoteness from the sun — even Jupiter being five times 
as far as the earth, and therefore receiving only one twenty-fifth 
of the light and heat that we receive per unit of surface — 
renders it almost impossible, even if other conditions were favour- 
able, that they should possess surface-temperatures adequate to 
the necessities of organic life. But their very low densities, com- 
bined with very large size, renders it certain that they none of 
them have a solidified surface, or even the elements from which 
such a surface could be formed. 

It is supposed that Jupiter and Saturn, as well as Uranus and 
Neptune, retain a considerable amount of internal heat, but cer- 
tainly not sufficient to keep the metallic and other elements of 



THE ONLY HABITABLE PLANET 269 

which the sun and earth consist in a state of vapour, for if so they 
would be planetary stars and would shine by their own light. 
And if any considerable portion of their bulk consisted of these 
elements, whether in a solid or a liquid state, their densities would 
necessarily be much greater than that of the earth instead of 
very much less — Jupiter is under one-fourth the density of the 
earth, Saturn under an eighth, while Uranus and Neptune are 
of intermediate densities, though much less in bulk even than 
Saturn. 

It thus appears that the solar system consists of two groups 
of planets which differ widely from each other. The outer group 
of four very large planets are almost wholly gaseous, and prob- 
ably consist of the permanent gases — those which can only be 
liquefied or solidified at a very low temperature. In no other 
way can their small density combined with enormous bulk be 
accounted for. 

The inner group also of four planets are totally unlike the 
preceding. They are all of small size, the earth being the 
largest. They are all of a density roughly proportionate to 
their bulk. The earth is both the largest and the densest of 
the group ; not only is it situated at that distance from the sun 
which, through solar heat alone, allows water to remain in the 
liquid state over almost the whole of its surface, but it possesses 
numerous characteristics which secure a very equable tempera- 
ture, and which have secured to it very nearly the same tempera- 
ture during those enormous geological periods in which ter- 
restrial life has existed. We have already shown that no other 
planet possesses these characteristics now, and it is almost equally 



270 MAN'S PLACE IN THE UNIVERSE 

certain that they never have possessed them in the past, and never 

will possess them in the future. 

A LAST ARGUMENT FOR HABITABILITY OF THE PLANETS 

Although it has been admitted by the late Mr. Proctor and some 
other astronomers that most of the planets are not now habitable, 
yet, it is often urged, they may have been so in the past or may 
become so in the future. Some are now too hot, others are now 
too cold; some have now no water, others have too much; but 
all go through their appointed series of stages, and during some 
of these stages life may be or may have been possible. This 
argument, although vague, will appeal to some readers, and it 
may, therefore, be necessary to reply to it. This is the more 
necessary as it is still made use of by astronomers. In a criticism 
of my article in the Fortnightly Review, M. Camille Flammarion, 
of the Paris Observatory, dramatically remarks : " Yes, life is 
universal, and eternal, for time is one of its factors. Yesterday 
the moon, to-day the earth, to-morrow Jupiter. In space there 
are both cradles and tombs." * 

It is thus suggested that the moon was once inhabited, and 
that Jupiter will be inhabited in some remote future; but no 
attempt is made to deal with the essential physical conditions of 
these very diverse objects, rendering them not only now, but 
always, unfitted to develop and to maintain terrestrial or aerial 
life. This vague supposition — it can hardly be termed an argu- 
ment — as regards past or future adaptability for life, of all 
the planets and some of the satellites in the solar system, is, how- 
1 Knowledge, June, 1903. 



THE ONLY HABITABLE PLANET 271 

ever, rendered invalid by an equally general objection to which 
its upholders appear never to have given a moment's considera- 
tion; and as it is an objection which still further enforces the 
view as to the unique position of the earth in the solar system, it 
will be well to submit it to the judgment of our readers. 

LIMITATION OF THE SUN'S HEAT 

It is well known that there is, and has been for nearly half a 
century, a profound difference of opinion between geologists and 
physicists as to the actual or possible duration in years of life 
upon the earth. The geologists, being greatly impressed with 
the vast results produced by the slow processes of the wearing 
away of the rocks and the deposit of the material in seas or 
lakes, to be again upheaved to form dry land, and to be again 
carved out by rain and wind, by heat and cold, by snow and ice, 
into hills and valleys and grand mountain ranges ; and further, 
by the fact that the highest mountains in every part of the globe 
very often exhibit on their loftiest summits stratified rocks which 
contain marine organisms, and were therefore originally laid 
down beneath the sea ; and, yet again, by the fact that the loftiest 
mountains are often the most recent, and that these grand 
features of the earth's surface are but the latest examples of 
the action of forces that have been at work throughout all geo- 
logical time — studying all their lives the detailed evidences of 
all these changes, have come to the conclusion that they imply 
enormous periods only to be measured by scores or hundreds of 
millions of years. 



272 MAN'S PLACE IN THE UNIVERSE 

And the collateral study of fossil remains in the long series of 
rock-formations enforces this view. In the whole epoch of 
human history, and far back into prehistoric times during which 
man existed on the earth, although several animals have become 
extinct, yet there is no proof that any new one has been devel- 
oped. But this human era, so far as yet known, going back 
certainly to the glacial epoch and almost certainly to pre-glacial 
times, cannot be estimated at less than a million, some think even 
several million years ; and as there have certainly been some con- 
siderable alterations of level, excavation of valleys, deposits of 
great beds of gravel, and other superficial changes during this 
period, some kind of a scale of measurement of geological time 
has been obtained, by comparison with the very minute changes 
that have occurred during the historical period. This scale is 
admittedly a very imperfect one, but it is better than none at all ; 
and it is by comparing these small changes with the far greater 
ones which have occurred during every successive step backward 
in geological history that these estimates of geological time have 
been arrived at. They are also supported by the palaeontolo- 
gists, to whom the vast panorama of successive forms of life is 
an ever-present reality. Directly they pass into the latest stage 
of the Tertiary period — the Pliocene of Sir Charles Lyell — all 
over the world new forms of life appear which are evidently the 
forerunners of many of our still existing species; and as they 
go a little further back, into the Miocene, there are indications 
of a warmer climate in Europe, and large numbers of mammals 
resembling those which now inhabit the tropics, but of quite dis- 
tinct species and often of distinct genera and families. And 



THE ONLY HABITABLE PLANET 273 

here, though we have only reached to about the middle of the 
Tertiary period, the changes in the forms of life, in the climate, 
and in the land-surfaces are so great when compared with the 
very minute changes during the human epoch, as to require us 
to multiply the time elapsed many times over. Yet the whole of 
the Tertiary period, during which all the great groups of the 
higher animals were developed from a comparatively few gen- 
eralised ancestral forms, is yet the shortest by far of the three 
great geological periods — the Mesozoic or Secondary, having 
been much longer, with still vaster changes both in the earth's 
crust and in the forms of life; while the Palaeozoic or Primary, 
which carries us back to the earliest forms of life as represented 
by fossilised remains, is always estimated by geologists to be 
at least as long as the other two combined and probably very 
much longer. 

From these various considerations most geologists who have 
made any estimates of geological time from the period of the 
earliest fossiliferous rocks, have arrived at the conclusion that 
about 200 millions of years are required. But from the variety 
of the forms of life at this early period it is concluded that a very 
much greater duration is needed for the whole epoch of life. 
Speaking of the varied marine fauna of the Cambrian period, the 
late Professor Ramsay says : " In this earliest known varied life 
we find no evidence of its having lived near the beginning of the 
zoological series. In a broad sense, compared with what must 
have gone before, both biologically and physically, all the phe- 
nomena connected with this old period seem, to my mind, to be of 
quite a recent description; and the climates of seas and lands 



274 MAN'S PLACE IN THE UNIVERSE 

were of the very same kind as those the world enjoys at the pres- 
ent day." And Professor Huxley held very similar views when 
he declared : " If the very small differences which are observable 
between the crocodiles of the older Secondary formations and 
those of the present day furnish any sort of an approximation 
towards an estimate of the average rate of change among rep- 
tiles, it is almost appalling to reflect how far back in Palaeozoic 
times we must go before we can hope to arrive at that common 
stock from which the crocodiles, lizards, Ornithoscelida, and 
Plesiosauria, which had attained so great a development in the 
Triassic epoch, must have been derived." 

Now, in opposition to these demands of the geologists, in which 
they are almost unanimous, the most celebrated physicists, after 
full consideration of all possible sources of the heat of the sun, 
and knowing the rate at which it is now expending heat, declare, 
with complete conviction, that our sun cannot have existed as a 
heat-giving body for so long a period, and they would therefore 
reduce the time during which life can possibly have existed on 
the earth to about one-fourth of that demanded by geologists. 
In one of his latest articles, Lord Kelvin says : " Now we have 
irrefragable dynamics proving that the whole life of our sun 
as a luminary is a very moderate number of million years, prob- 
ably less than 50 million, possibly between 50 and 100 " (Phil. 
Mag., vol ii., Sixth Ser., p. 175, Aug., 1901). In my Island 
Life ( Chapter X ) I have myself given reasons for thinking that 
both the stratigraphical and biological changes may have gone 
on more quickly than has been supposed, and that geological 
time (meaning thereby the time during which the development 



THE ONLY HABITABLE PLANET 275 

of life upon the earth has been going on) may be reduced so as 
possibly to be brought within the maximum period allowed by 
physicists ; but there will certainly be no time to spare, and any 
planets dependent on our sun, whose period of habitability is 
either past, or to come, cannot possibly have, or have had, suffi- 
cient time for the necessarily slow evolution of the higher life- 
forms. Again, all physicists hold that the sun is now cooling, 
and that its future life will be much less than its past. In a 
lecture at the Royal Institution (published in Nature Series, in 
1889), Lord Kelvin says: "It would, I think, be exceedingly 
rash to assume as probable anything more than twenty million 
years of the sun's light in the past history of the earth, or to 
reckon more than five or six million years of sunlight for time to 
come." 

These extracts serve to show that, unless either geologists or 
physicists are very far from any approach to accuracy in their 
estimates of past or future age of the sun, there is very great 
difficulty in bringing them into harmony or in accounting for 
the actual facts of the geological history of the earth and of the 
whole course of life-development upon it. We are, therefore, 
again brought to the conclusion that there has been, and is, no 
time to spare; that the whole of the available past life-period 
of the sun has been utilised for life-development on the earth, 
and that the future will be not much more than may be needed 
for the completion of the grand drama of human history, and 
the development of the full possibilities of the mental and moral 
nature of man. 

We have here, then, a very powerful argument, from a dif- 



276 MAN'S PLACE IN THE UNIVERSE 

f erent point of view than any previously considered, for the con- 
clusion that man's place in the solar system is altogether unique, 
and that no other planet either has developed or can develop such 
a full and complete life-series as that which the earth has actually 
developed. Even if the conditions had been more favourable 
than they are seen to be on other planets, Mercury, Venus, and 
Mars could not possibly have preserved equability of conditions 
long enough for life-development, since for unknown ages they 
must have been passing slowly towards their present wholly un- 
suitable conditions; while Jupiter and the planets beyond him, 
whose epoch of life-development is supposed to be in the remote 
future when they shall have slowly cooled down to habitability, 
will then be still more faintly illuminated and scantily warmed by 
a rapidly cooling sun, and may thus become, at the best, globes of 
solid ice. This is the teaching of science — of the best science of 
the twentieth century. Yet we find even astronomers who, more 
than any other exponents of science, should give heed to the teach- 
ings of the sister sciences to which they owe so much, indulging 
in such rhapsodies as the following : " In our solar system, this 
little earth has not obtained any special privileges from Nature, 
and it is strange to wish to confine life within the circle of ter- 
restrial chemistry." And again : " Infinity encompasses us on 
all sides, life asserts itself, universal and eternal, our existence 
is but a fleeting moment, the vibration of an atom in a ray of the 
sun, and our planet is but an island floating in the celestial archi- 
pelago, to which no thought will ever place any bounds." * 
In place of such " wild and whirling words," I have en- 
1 M. Camille Flammarion, in Knowledge, June, 1903. 



THE ONLY HABITABLE PLANET 277 

deavoured to state the sober conclusions of the best workers and 
thinkers as to the nature and origin of the world in which we 
live, and of the universe which on all sides surrounds us. I 
leave it to my readers to decide which is the most trustworthy 
guide. 



CHAPTER XV 

THE STARS HAVE THEY PLANETARY SYSTEMS? ARE THEY 

BENEFICIAL TO US? 

Most of the writers on the Plurality of Worlds, from Fontenelle 
to Proctor, taking into consideration the enormous number of 
the stars and their apparent uselessness to our world, have as- 
sumed that many of them must have systems of planets circling 
round them, and that some of these planets, at all events, must 
possess inhabitants, some, perhaps, lower, but others no doubt 
higher than ourselves. One of our well-known modern astrono- 
mers, writing only ten years ago, adopts the same view. He 
says : " The suns which we call stars were clearly not created for 
our benefit. They are of very little practical use to the earth's 
inhabitants. They give us very little light ; an additional small 
satellite — one considerably smaller than the moon — would have 
been much more useful in this respect than the millions of stars 
revealed by the telescope. They must therefore have been 
formed for some other purpose. . . . We may therefore con- 
clude, with a high degree of probability, that the stars — at least 
those with spectra of the solar type — form centres of planetary 
systems somewhat similar to our own." x The author then dis- 
cusses the conditions necessary for life analogous to that of our 

1 The Worlds of Space, by J. E. Gore, chapter iii. 

278 



THE STARS IN RELATION TO LIFE 279 

earth, as regards temperature, rotation, mass, atmosphere, water, 
etc., and he is the only writer I have met with who has considered 
these conditions; but he touches on them very briefly, and he 
arrives at the conclusion that, in the case of the stars of solar 
type, it is probable that one planet, situated at a proper dis- 
tance, would be fitted to support life. He estimates roughly 
that there are about ten million stars of this type, that is, closely 
resembling our sun, and that if only one in ten of these has a 
planet at the proper distance and properly constituted in other 
respects, there will be one million worlds fitted for the support of 
animal life. He therefore concludes that there are probably 
many stars having life-bearing planets revolving round them. 

There are, however, many considerations not taken account of 
by this writer which tend to reduce very considerably the above 
estimate. It is now known that immense numbers of the stars 
of smaller magnitudes are nearer to us than are the majority of 
the stars of the first and second magnitudes, so that it is probable 
that these, as well as a considerable proportion of the very faint 
telescopic stars, are really of small dimensions. We have evi- 
dence that many of the brightest stars are much larger than our 
sun, but there are probably ten times as many that are much 
smaller. We have seen that the whole of the past light and heat- 
giving duration of our sun has, according to the best authorities, 
been only just sufficient for the development of life upon the 
earth. But the duration of a sun's heat-giving power will 
depend mainly upon its mass, together with its constituent ele- 
ments. Suns which are much smaller than ours are, therefore, 
from that cause alone, unsuited to give adequate light and heat 



280 MAN'S PLACE IN THE UNIVERSE 

for a sufficient time, and with sufficient uniformity, for life- 
development on planets, even if they possess any at the right 
distance, and with the extensive series of nicely adjusted condi- 
tions which I have shown to be necessary. 

Again, we must, probably, rule out as unfitted for life-devel- 
opment the whole region of the Milky Way, on account of the 
excessive forces there in action, as shown by the immense size 
of many of the stars, their enormous heat-giving power, the 
crowding of stars and nebulous matter, the great number of star- 
clusters, and, especially, because it is the region of " new stars," 
which imply collisions of masses of matter sufficiently large to 
become visible from the immense distance we are from them, but 
yet excessively small as compared with suns the duration of whose 
light is to be measured by millions of years. Hence the Milky 
Way is the theatre of extreme activity and motion; it is com- 
paratively crowded with matter undergoing continual change, 
and is therefore not sufficiently stable for long periods to be at 
all likely to possess habitable worlds. 

We must, therefore, limit our possible planetary systems suit- 
able for life-development, to stars situated inside the circle of the 
Milky Way and far removed from it — that is, to those com- 
posing the solar cluster. These have been variously estimated 
to consist of a few hundred or many thousand stars — at all events 
to a very small number as compared with the " hundreds of 
millions " in the whole stellar universe. But even here we find 
that only a portion are probably suitable. Professor Newcomb 
arrives at the conclusion — as have some other astronomers — that 
the stars in general have a much smaller mass in proportion to 



THE STARS IN RELATION TO LIFE 281 

the light they give than our sun has; and, after an elaborate 
discussion, he finally concludes that the brighter stars are, on 
the average, much less dense than our sun. In all probability, 
therefore, they cannot give light and heat for so long a period, 
and as this period in the case of our sun has only been just sum*-, 
cient, the number of suns of the solar type and of a sufficient mass 
may be very limited. Yet further, even among stars having a 
similar physical constitution to our sun, and of an equal or 
greater mass, only a portion of their period of luminosity would 
be suitable for the support of planetary life. While they are 
in process of formation by accretions of solid or gaseous masses, 
they would be subject to such fluctuations of temperature, and 
to such catastrophic outbursts when any larger mass than usual 
was drawn towards them, that the whole of this period — perhaps 
by far the longest portion of their existence — must be left out 
of the account of planet-producing suns. Yet all these are to 
us stars of various degrees of brilliancy. It is almost certain 
that it is only when the growth of a sun is nearly completed, and 
its heat has attained a maximum, that the epoch of life-develop- 
ment is likely to begin upon any planets it may possess at the 
most suitable distance, and upon which all the requisite condi- 
tions should be present. 

It may be said that there are great numbers of stars beyond 
our solar cluster and yet within the circle of the Milky Way, as 
well as others towards the poles of the Milky Way, which I have 
not here referred to. But of these regions very little is known, 
because it is impossible to tell whether stars in these directions 
are situated in the outer portion of the solar cluster, or in the 



282 MAN'S PLACE IN THE UNIVERSE 

regions beyond it. Some astronomers appear to think that these 
regions may be nearly empty of stars, and I have endeavoured 
to represent what seems to be the general view on this very diffi- 
cult subject in the two diagrams of the stellar universe at p. 296. 
The regions beyond our cluster and above or below the plane of 
the Milky Way are those where the small irresolvable nebulae 
abound, and these may indicate that sun-formation is not yet 
active in those regions. The two charts of Nebulas and Clusters 
at the end of the volume illustrate, and perhaps tend to support 
this view. 

DOUBLE AND MULTIPLE STAR SYSTEMS 

We have already seen, in our sixth chapter, how rapid and ex- 
traordinary has been the discovery of what are termed spectro- 
scopic binaries — pairs of stars so close together as to appear like 
a single star in the most powerful telescopes. The systematic 
search for such stars has only been carried on for a few years, 
yet so many have been already found, and their numbers are in- 
creasing so rapidly, as to quite startle astronomers. One of 
the chief workers in this field, Professor Campbell of the Lick 
Observatory, has stated his opinion that, as accuracy of measure- 
ment increases, these discoveries will go on till — " the star that is 
not a spectroscopic binary will prove to be the rare exception," 
— and other astronomers of eminence have expressed similar 
views. But these close revolving star-systems are generally ad- 
mitted to be out of the category of life-producing suns. The 
tidal disturbances mutually produced must be enormous, and 
this must be inimical to the development of planets, unless they 






THE STARS IN RELATION TO LIFE 283 

were very close to each sun, and thus in the most unfavourable 
position for life. 

We thus see that the result of the most recent researches 
among the stars is entirely opposed to the old idea that the count- 
less myriads of stars all had planets circulating round them, and 
that the ultimate purpose of their existence was, that they should 
be supporters of life, as our sun is the supporter of life upon 
the earth. So far is this from being the case, that vast numbers 
of stars have to be put aside as wholly unfitted for such a pur- 
pose ; and when by successive eliminations of this nature we have 
reduced the numbers which may possibly be available to a few 
millions, or even to a few thousands, there comes the last startling 
discovery, that the entire host of stars is found to contain binary 
systems in such rapidly increasing numbers, as to lead some of 
the very first astronomers of the day to the conclusion that single 
stars may some day be found to be the rare exception ! But 
this tremendous generalisation would, at one stroke, sweep away 
a large proportion of the stars which other successive disqualifi- 
cations had spared, and thus leave our sun, which is certainly 
single, and perhaps two or three companion orbs, alone among 
the starry host as possible supporters of life on some one of the 
planets which circulate around them. 

But we do not really know that any such suns exist. If they 
exist we do not know that they possess planets. If any do pos- 
sess planets these may not be at the proper distance, or be of the 
proper mass, to render life possible. If these primary conditions 
should be fulfilled, and if there should possibly be not only one 
or two, but a dozen or more that so far fulfil the first few condi- 



284 MAN'S PLACE IN THE UNIVERSE 

tions which are essential, what probability is there that all the 
other conditions, all the other nice adaptations, all the delicate 
balance of opposing forces that we have found to prevail upon 
the earth, and whose combination here is due to exceptional con- 
ditions which exist in the case of no other known planet — should 
all be again combined in some of the possible planets of these 
possibly existing suns? 

I submit that the probability is now all the other way. So 
long as we could assume that all the stars might be, in all essen- 
tials, like our sun, it seemed almost ludicrous to suppose that our 
sun alone should be in a position to support life. But when we 
find that enormous classes like the gaseous stars of small density, 
the solar stars while increasing in size and temperature, the stars 
which are much smaller than our sun, the nebulous stars, prob- 
ably all the stars of the Milky Way, and lastly, that enormous 
class of spectroscopic doubles — veritable Aaron's rods which 
threaten to swallow up all the rest — that all these are for various 
reasons unlikely to have attendant planets adapted to develop 
life, then the probabilities seem to be enormously against there 
being any considerable number of suns possessing attendant 
habitable earths. Just as the habit ability of all the planets and 
larger satellites, once assumed as so extremely probable as to 
amount almost to a certainty, is now generally given up, so that 
in speculating on life in stellar systems Mr. Gore assumes that 
only one planet to each sun can be habitable ; in like manner it 
may, and I believe will, turn out, that of all the myriad stars, 
the more we learn about them, the smaller and smaller will become 
the scanty residue which, with any probability, we can suppose 



THE STARS IN RELATION TO LIFE 285 

to illuminate and vivify habitable earths. And when with this 
scanty probability we combine the still scantier probability that 
any such planet will possess simultaneously, and for a sufficiently 
long period, all the highly complex and delicately balanced condi- 
tions known to be essential for a full life-development, the con- 
ception that on this earth alone has such development been com- 
pleted will not seem so wildly improbable a conjecture as it has 
hitherto been held to be. 

ARE THE STARS BENEFICIAL TO US? 

When I suggested in my first publication on this subject that 
some emanations from the stars might be beneficial or injurious, 
and that a central position might be essential in order to render 
these emanations equable, one of my astronomical critics laughed 
the idea to scorn, and declared that " we might wander into outer 
space without losing anything more serious than we lose when 
the night is cloudy and we cannot see the stars." * How my 
critic knows that this is so he does not tell us. He states it posi- 
tively, with no qualification, as if it were an established fact. It 
may be as well to inquire, therefore, if there is any evidence 
bearing upon the point at issue. 

Astronomers are so fully occupied with the vast number and 
variety of the phenomena presented by the stellar universe and 
the various difficult problems arising therefrom, that many lesser 
but still interesting inquiries have necessarily received little at- 
tention. Such a minor problem is the determination of how much 
1 The Fortnightly Review, April, 1903, p. 60. 



286 MAN'S PLACE IN THE UNIVERSE 

heat or other active radiation we receive from the stars ; yet a few 
observations have been made with results that are of considerable 
interest. 

In the years 1900 and 1901 Mr. E. F. Nichols of the Yerkes 
Observatory made a series of experiments with a radiometer of 
special construction, to determine the heat emitted by certain 
stars. The result arrived at was, that Vega gave about 
-g-oir.o i o.tot of the heat of a candle at one metre distance, and 
Arcturus about 2.2 times as much. 



1896 



1895 



Source of Light 



Candle at 10 feet distance, 
Betelguese (0 9 mag.), . . 
Aldebaran (11 mag.), . . 
Procyon (0 5 mag.), . . 
Alpha Cygni (1 3 mag.), . 
Polaris (2 1 mag.), . . . 

1 volt 

Arcturus (0 3 mag.), . . 
Vega (0-1 mag.), . . . 
Candle at 10 feet, . . . 



Deflection 

in 
Millimetres 



18 70 
12 80 
5-21 
4-89 
4-90 
310 



432 00 

8-2 

11-5 

81 



Light 

in 

Candles 



0-685 
0-279 
261 
0-262 
0166 



01 



E. M. F. 

Volts 



026 
0012 
0011 
0011 
007 



0019 
026 



N. B. — The standard candle shone directly on the cell, whereas the star's 
light was concentrated by a 2- foot mirror. 



In 1895 and 1896 Mr. G. M. Minchin made a series of experi- 
ments on the Electrical Measurement of Starlight, by means of 
a photo-electric cell of peculiar construction which is sensitive 
to the whole of the rays in the spectrum, and also to some of the 
ultra-red and ultra-violet rays. Combined with this was a very 



THE STARS IN RELATION TO LIFE 287 

delicate electrometer. The telescope employed to concentrate the 
light was a reflector of two feet aperture. Mr. Minchin was as- 
sisted in the experiments by the late Professor G. F. Fitzgerald, 
F. R. S., of Trinity College, Dublin, which may be considered a 
guarantee of the accuracy of the observations. The foregoing 
are the chief results obtained. 

The sensitive surface on which the light of the stars was con- 
centrated was sV inch in diameter. We must therefore diminish 
the amount of candle light in this table in the proportion of the 
square of the diameter of the mirror (in 2V ths of an inch) to 
one, or -gro.VFo • If we make the necessary reduction in the 
case of Vega, and also equalise the distance at which the candle 
was placed, we find the following result : 



Observer Star Candle power at 10 ft. 

Minchin Vega tihf.W 

Nichols yy.Tnrfr.Tnnr 



This enormous difference in the result is no doubt largely due 
to the fact that Mr. Nichols's apparatus measured heat alone, 
whereas Mr. Minchin's cell measured almost all the rays. And 
this is further shown by the fact that, whereas Mr. Nichols found 
Arcturus a red star, hotter than Vega a white one, Mr. Minchin, 
measuring also the light-giving and some of the chemical rays, 
found Vega considerably more energetic than Arcturus. These 
comparisons also suggest that other modes of measurement might 
give yet higher results, but it will no doubt be urged that such 
minute effects must necessarily be quite inoperative upon the 
organic world. 



288 MAN'S PLACE IN THE UNIVERSE 

There are, however, some considerations which tend the other 
way. Mr. Minchin remarks on the unexpected fact that Betel- 
guese produces more than double the electrical energy of 
Procyon, a much brighter star. This indicates that many of the 
stars of smaller visual magnitudes may give out a large amount 
of energy, and it is this energy, which we now know can take 
many strange and varied forms, that would be likely to influence 
organic life. And as to the quantity being too minute to have any 
effect, we know that the excessively minute amount of light from 
the very smallest telescopic stars produces such chemical changes 
on a photographic plate as to form distinct images, with com- 
paratively small lenses or reflectors and with an exposure of two 
or three hours. And if it were not that the diffused light of the 
surrounding sky also acts upon the plate and blurs the faint 
images, much smaller stars could be photographed. 

We know that not all the rays but a portion only are capable 
of producing these effects; we know also that there are many 
kinds of radiation from the stars, and probably some yet undis- 
covered comparable with the X-rays and other new forms of 
radiation. We must also remember the endless variety and the 
extreme instability of the protoplasmic products in the living 
organism, many of which are perhaps as sensitive to special rays 
as is the photographic plate. And we are not here limited to 
action for a few minutes or a few hours, but throughout the 
whole night and day, and continued whenever the sky is clear 
for months or years. Thus the cumulative effect of these very 
weak radiations may become important. It is probable that 
their action would be most influential on plants, and here we find 



THE STARS IN RELATION TO LIFE 289 

all the conditions requisite for its accumulation and utilisation 
in the large amount of leaf-surface exposed to it. A large tree 
must present some hundreds of superficial feet of receptive sur- 
face, while even shrubs and herbs often have a leaf-area of 
greater superficial extent than the object-glasses of our largest 
telescopes. Some of the highly complex chemical processes that 
go on in plants may be helped by these radiations, and their 
action would be increased by the fact that, coming from every 
direction over the whole surface of the heavens, the rays from the 
stars would be able to reach and act upon every leaf of the 
densest masses of foliage. The large amount of growth that 
takes place at night may be in part due to this agency. 

Of course all this is highly speculative ; but I submit, in view 
of the fact that the light of the very faintest stars does produce 
distinct chemical changes, that even the very minute heat-effects 
are measurable, as well as the electro-motive forces caused by 
them ; and further, that when we consider the millions, perhaps 
hundreds of millions of stars, all acting simultaneously on any 
organism which may be sensitive to them, the supposition that 
they do produce some effect, and possibly a very important effect, 
is not one to be summarily rejected as altogether absurd and not 
worth inquiring into. 

It is not, however, these possible direct actions of the stars 
upon living organisms to which I attach much weight as regards 
our central position in the stellar universe. Further considera- 
tion of the subject has convinced me that the fundamental im- 
portance of that position is a physical one, as has already been 
suggested by Sir Norman Lockyer and some other astronomers. 



290 MAN'S PLACE IN THE UNIVERSE 

Briefly, the central position appears to be the only one where 
suns can be sufficiently stable and long-lived to be capable of 
maintaining the long process of life-development in any of the 
planets they may possess. This point will be further developed 
in the next and concluding chapter. 



CHAPTER XVI 

STABILITY OF THE STAR-SYSTEM : IMPORTANCE OF OUR CENTRAL 
POSITION : SUMMARY AND CONCLUSION 

One of the greatest difficulties with regard to the vast system 
of stars around us is the question of its permanence and stability, 
if not absolutely and indefinitely, yet for periods sufficiently long 
to allow for the many millions of years that have certainly been 
required for our terrestrial life-development. This period, in the 
case of the earth, as I have sufficiently shown, has been character- 
ised throughout by extreme uniformity, while a continuance of 
that uniformity for a few millions of years in the future is 
almost equally certain. 

But our mathematical astronomers can find no indications of 
such stability of the stellar universe as a whole, if subject to 
the law of gravitation alone. In reply to some questions on this 
point, my friend Professor George Darwin writes as follows: 
" A symmetrical annular system of bodies might revolve in a 
circle with or without a central body. Such a system would be 
unstable. If the bodies are of unequal masses and not sym- 
metrically disposed, the break-up of the system would probably 
be more rapid than in the ideal case of symmetry." 

This would imply that the great annular system of the Milky 
Way is unstable. But if so, its existence at all is a greater mys- 

291 



292 MAN'S PLACE IN THE UNIVERSE 

tery than ever. Although in detail its structure is very irregu- 
lar, as a whole it is wonderfully symmetrical; and it seems quite 
impossible that its generally circular ring-like form can be the 
result of the chance aggregation of matter from any pre-exist- 
ing different form. Star-clusters are equally unstable, or, rather, 
nothing is known or can be predicated about their stability or 
instability, according to Professors Newcomb and Darwin. 

Mr. E. T. Whittaker (Secretary to the Royal Astronomical 
Society), to whom Professor G. Darwin sent my questions, 
writes : " I doubt whether the principal phenomena of the stellar 
universe are consequences of the law of gravitation at all. I 
have been working myself at spiral nebulae, and have got a first 
approximation to an explanation — but it is electro-dynamical 
and not gravitational. In fact, it may be questioned whether, 
for bodies of such tremendous extent as the Milky Way or 
nebulae, the effect which we call gravitation is given by New- 
ton's law ; just as the ordinary formulae of electrostatic attraction 
break down when we consider charges moving with very great 
velocities." 

Accepting these statements and opinions of two mathemati- 
cians who have paid special attention to similar problems, we 
need not limit ourselves to the laws of gravitation as having de- 
termined the present form of the stellar universe; and this is 
the more important because we may thus escape from a conclusion 
which many astronomers seem to think inevitable, viz., that the 
observed proper motions of the stars cannot be explained by the 
gravitative forces of the system itself. In Chapter VIII of this 
work I have quoted Professor Newcomb's calculation as to the 



STABILITY OF STELLAR UNIVERSE 293 

effect of gravitation in a universe of 100 million stars, each five 
times the mass of our sun, and spread over a sphere which it 
would take light 30,000 years to cross ; then, a body falling from 
its outer limits to the centre could at the utmost acquire a velocity 
of twenty-five miles a second; and therefore, any body in any 
part of such a universe having a greater velocity would pass 
away into infinite space. Now, as several stars have, it is be- 
lieved, much more than this velocity, it follows not only that 
they will inevitably escape from our universe, but that they do 
not belong to it, as their great velocity must have been acquired 
elsewhere. This seems to have been the idea of the astronomer 
who stated that, even at the very moderate speed of our sun, we 
should in five million years be deep in the actual stream of the 
Milky Way. To this I have already sufficiently replied; but 
I now wish to bring before my readers an excellent illustration 
of the importance of the late Professor Huxley's remark, that 
the results you got out of the " mathematical mill " depend en- 
tirely on what you put into it. 

In the Philosophical Magazine (January, 1902) is a remark- 
able article by Lord Kelvin, in which he discusses the very same 
problem as that which Professor Newcomb had discussed at a 
much earlier date, but, starting from different assumptions, 
equally based on ascertained facts and probabilities deduced from 
them, brings out a very different result. 

Lord Kelvin postulates a sphere of such a radius that a star 
at its confines would have a parallax of one-thousandth part of 
a second (0".001), equivalent to 3215 light-years. Uniformly 
distributed through this sphere there is matter equal in mass to 



294 MAN'S PLACE IN THE UNIVERSE 

1000 million suns like ours. If this matter becomes subject to 
gravitation, it all begins to move at first with almost infinite 
slowness, especially near its centre; but nevertheless, in twenty- 
five million years many of these suns would have acquired veloc- 
ities of from twelve to twenty miles a second, while some would 
have less and some probably more than seventy miles a second. 
Now such velocities as these agree generally with the measured 
velocities of the stars, hence Lord Kelvin thinks there may be 
as much matter as 1000 million suns within the above-named dis- 
tance. He then states that if we suppose there to be 10,000 
million suns within the same sphere, velocities would be produced 
very much greater than the known star- velocities ; hence it is 
probable that there is very much less matter than 10,000 million 
times the sun's mass. He also states that if the matter were not 
uniformly distributed within the sphere, then, whatever was the 
irregularity, the acquired motions would be greater ; again indi- 
cating that the 1000 million suns would be ample to produce the 
observed effects of stellar motion. He then calculates the aver- 
age distance apart of each of the 1000 million stars, which he 
finds to be about 300 millions of millions of miles. Now the 
nearest star to our sun is about twenty-six million million of miles 
distant, and, as the evidence shows, is situated in the denser part 
of the solar cluster. This gives ample allowance for the com- 
parative emptiness of the space between our cluster and the 
Milky Way, as well as of the whole region towards the poles of 
the Milky Way (as shown by the diagrams in Chapter IV), while 
the comparative density of extensive portions of the Galaxy itself 
may serve to make up the average. 



STABILITY OF STELLAR UNIVERSE 295 

Now, previous writers have come to a different conclusion from 
the same general line of argument, because they have started 
with different assumptions. Professor Newcomb, whose state- 
ment made some years back is usually followed, assumed 100 mill- 
ion stars each five times as large as our sun, equal to 500 million 
suns in all, and he distributed them equally throughout a sphere 
30,000 light-years in diameter. Thus he has half the amount 
of matter assumed by Lord Kelvin, but nearly five times the 
extent, the result being that gravity could only produce a maxi- 
mum speed of twenty -five miles a second ; whereas on Lord Kel- 
vin's assumption a maximum speed of seventy miles a second would 
be produced, or even more. By this latter calculation we find no 
insuperable difficulty in the speed of any of the stars being 
beyond the power of gravitation to produce, because the rates 
here given are the direct results of gravitation acting on bodies 
almost uniformly distributed through space. Irregular dis- 
tribution, such as we see everywhere in the universe, might lead 
to both greater and less velocities; and if we further take ac- 
count of collisions and near approaches of large masses resulting 
in explosive disruptions, we might have almost any amount of 
motion as the result, but as this motion would be produced by 
gravitation within the system, it could equally well be controlled 
by gravitation. 

In order that my readers may better understand the calcula- 
tions of Lord Kelvin, and also the general conclusions of astrono- 
mers as to the form and dimensions of the stellar universe, I have 
drawn two diagrams, one showing a plan on the central plane 
of the Milky Way, the other a section through its poles. Both 



296 MAN'S PLACE IN THE UNIVERSE 

DIAGRAM OF STELLAR UNIVERSE (Plan) 










r 






:.^K«r.->y:r ' 



fc- JH WPZightYecCrs I20O-'li<j7vlrYecifr 







:■;, i -/ ■■''■'■■-■. /-v^w 






1. Central part of Solar Cluster. 

2. Sun's Orbit (black spot). 

3. Outer limit of Solar Cluster. 

4. Milky Way. 



are on the same scale, and they show the total diameter across 
the Milky Way as being 3600 light-years, or about half that 



STABILITY OF STELLAR UNIVERSE 297 

postulated by Lord Kelvin for his hypothetical universe. I do 
this because the dimensions given by him are those which are 
sufficient to lead to motions near the centre such as the stars now 

DIAGRAM OF STELLAR UNIVERSE (Section) 
.... Pole of Galaxy. » 

•-•'^v-^-v :/"!/•/ ■'•'•'••'••'•'• Nebulae abundant , --\ '/'''■•••/:' '••/.' ''•'- . 






Nebtilee abundant 



Section through Poles of Milky Way. 

possess in a minimum period of twenty-five million years after 
the initial arrangement he supposes, at which later epoch which 
we are now supposed to have reached, the whole system would 
of course be greatly reduced in extent by aggregations towards 



298 MAN'S PLACE IN THE UNIVERSE 

and near the centre. These dimensions also seem to accord suffi- 
ciently with the actual distances of stars as yet measured. The 
smallest parallax which has been determined with any certainty, 
according to Professor Newcomb's list, is that of Gamma Cas- 
siopeia?, which is one-hundredth of a second (0".01), while Lord 
Kelvin gives none smaller than 0".02, and these will all be in- 
cluded within the solar cluster as I have shown it. 

It must be clearly understood that these two illustrations are 
merely diagrams to show the main features of the stellar universe 
according to the best information available, with the propor- 
tionate dimensions of these features, so far as the facts of the 
distribution of the stars and the views of those astronomers who 
have paid most attention to the subject can be harmonised. Of 
course it is not suggested that the whole arrangement is so regu- 
lar as here shown, but an attempt has been made by means of 
the dotted shading to represent the comparative densities of the 
different portions of space around us, and a few remarks on this 
point may be needed. 

The solar cluster is shown very dense at the central portion, 
occupying one-tenth of its diameter, and it is near the outside 
of this dense centre that our sun is supposed to be situated. Be- 
yond this there seems to be almost a vacuity, beyond which again 
is the outer portion of the cluster consisting of comparatively 
thinly scattered stars, thus forming a kind of ring-cluster, 
resembling in shape the beautiful ring-nebula in Lyra, as has 
been suggested by several astronomers. There is some direct 
evidence for this ring- form. Professor Newcomb in his recent 
book on The Stars gives a list of all stars of which the parallax 



STABILITY OF STELLAR UNIVERSE 299 

is fairly well known. These are sixty-nine in number; and on 
arranging them in the order of the amount of their parallax, 
I find that no less than thirty-five of them have parallaxes be- 
tween 0".l and0".4 of a second, thus showing that they constitute 
part of the dense central mass ; while three others, from 0".4 to 
0".75, indicate those which are our closest companions at the 
present time, but still at an enormous distance. Those which 
have parallaxes of less than the tenth and down to one-hundredth 
of a second are only thirty-one in all ; but as they are spread over 
a sphere ten times the diameter, and therefore a thousand times 
the cubic content of the sphere containing those above one-tenth 
of a second, they ought to be immensely more numerous even if 
very much more thinly scattered. The interesting point, how- 
ever, is, that till we get down to a parallax of 0".06, there are 
only three stars as yet measured, whereas those between 0".2 and 
0".6, an equal range of parallax, are twenty-six in number, and 
as these are scattered in all directions they indicate an almost 
vacant space followed by a moderately dense outer ring. 

In the enormous space between our cluster and the Milky Way, 
and also above and below its plane to the poles of the Galaxy, 
stars appear to be very thinly scattered, perhaps more densely in 
the plane of the Milky Way than above and below it where the 
irresolvable nebulse are so numerous ; and there may not improb- 
ably be an almost vacant space beyond our cluster for a con- 
siderable distance, as has been supposed, but this cannot be known 
till some means are discovered of measuring parallaxes of from 
one-hundredth to one five-hundredth of a second. 

These diagrams also serve to indicate another point of con- 



300 MAN'S PLACE IN THE UNIVERSE 

siderable importance to the view here advocated. By placing 
the solar system towards the outer margin of the dense central 
portion of the solar cluster (which may very possibly include a 
large proportion of dark stars and thus be much more dense 
towards the centre than it appears to us), it may very well 
be supposed to revolve, with the other stars composing it, around 
the centre of gravity of the cluster, as the force of gravity 
towards that centre might be perhaps twenty or a hundred times 
greater than towards the very much less dense and more remote 
outer portions of the cluster. The sun, as indicated on the dia- 
grams, is about thirty light-years from that centre, correspond- 
ing to a parallax of a little more than one-tenth of a second, and 
an actual distance of 190 millions of millions of miles, equal to 
about 70,000 times the distance of the sun from Neptune. Yet 
we see that this position is so little removed from the exact centre 
of the whole stellar universe, that if any beneficial influences are 
due to that central position in regard to the Galaxy, it will 
receive them perhaps to as full an extent as if situated at the 
actual centre. But if it is situated as here shown, there is no 
further question as to its proper motion carrying it from one 
side to the other of the Milky Way in less time than has been 
required for the development of life upon the earth. And if 
the solar cluster is really sub-globular, and sufficiently condensed 
to serve as a centre of gravity for the whole of the stars of the 
cluster to revolve around, all the component stars which are not 
situated in the plane of its equator ( and that of the Milky Way ) 
must revolve obliquely at various angles up to an angle of 90°. 
These numerous diverging motions, together with the motions 



IMPORTANCE OF CENTRAL POSITION 301 

of the nearer stars outside the cluster, some of which may re- 
volve round other centres of gravity made up largely of dark 
bodies, would perhaps sufficiently account for the apparent ran- 
dom motions of so many of the stars. 

UNIFORM HEAT-SUPPLY DUE TO CENTRAL POSITION 

We now come to a point of the greatest interest as regards the 
problem we are investigating. We have seen how great is the 
difference in the estimates of geologists and those of physicists 
as to the time that has elapsed during the whole development of 
life. But the position we have now found for the sun, in the 
outer portion of the central star-cluster may afford a clue to this 
problem. What we require is, some mode of keeping up the sun's 
heat during the enormous geological periods in which we have 
evidence of a wonderful uniformity in the earth's temperature, 
and therefore in the sun's heat-emission. The great central 
ring-cluster with its condensed central mass, which presumably 
has been forming for a much longer period than our sun has been 
giving heat to the earth, must during all this time have been 
exerting a powerful attraction on the diffused matter in the 
spaces around it, now apparently almost void as compared with 
what they may have been. Some scanty remnants of that matter 
we see in the numerous meteoric swarms which have been drawn 
into our system. A position towards the outside of this central 
aggregation of suns would evidently be very favorable for the 
growth by accretion of any considerable mass. The enormous 
distance apart of the outer components (the outer ring) of the 



302 MAN'S PLACE IN THE UNIVERSE 

cluster would allow a large amount of the inflowing meteoritic 
matter to escape them, and the larger suns situated near the 
surface of the inner dense cluster would draw to themselves the 
greater part of this matter. 1 The various planets of our system 
were no doubt built up from a portion of the matter that flowed 
in near the plane of the ecliptic, but much of that which came 
from all other directions would be drawn towards the sun itself 
or to its neighbouring suns. Some of this would fall directly 
into it; other masses coming from different directions and col- 
liding with each other would have their motion checked, and thus 
again fall into the sun ; and so long as the matter falling in were 
not in too large masses, the slow additions to the sun's bulk and 
increase of its heat would be sufficiently gradual to be in no way 
prejudicial to a planet at the earth's distance. 

The main point I wish to suggest here is, that by far the 
greater portion of the matter of the whole stellar universe has, 
either through gravitation or in combination with electrical 
forces, as suggested by Mr. Whittaker, become drawn together 
into the vast ring-formed system of the Milky Way, which is, 
presumably, slowly revolving, and has thus been checked in its 
original inflow towards the centre of mass of the stellar universe. 

1 Since writing this chapter I have seen a paper by Luigi d'Auria dealing 
mathematically with " Stellar Motion," etc., and am pleased to see that, 
from quite different considerations, he has found it necessary to place the 
solar system at a distance from the centre not very much more remote than 
the position I have given it. He says: "We have good reasons to suppose 
that the solar system is rather near the centre of the Milky Way, and as 
this centre would, according to our hypothesis, coincide with the centre of the 
Universe, the distance of 159 light years assumed is not too great, nor can 
it be very much smaller."— Journal of the Franklin Institute, March, 1903. 



IMPORTANCE OF CENTRAL POSITION 303 

It has also probably drawn towards itself the adjacent por- 
tions of the scattered material in the spaces around it in all 
directions. 

Had the vast mass of matter postulated by Lord Kelvin ac- 
quired no motion of revolution, but have fallen continuously 
towards the centre of mass, the motions developed when the more 
distant bodies approached that centre would have been extremely 
rapid; while, as they must have fallen in from every direction, 
they would have become more and more densely aggregated, and 
collisions of the most catastrophic nature would frequently have 
occurred, and this would have rendered the central portion of 
the universe the least stable and the least fitted to develop 
life. 

But, under the conditions that actually prevail, the very reverse 
is the case. The quantity of matter remaining between our 
cluster and the Milky Way being comparatively small, the ag- 
gregation into suns has gone on more regularly and more slowly. 
The motions acquired by our sun and its neighbours have been 
rendered moderate by two causes : ( 1 ) their nearness to the centre 
of the very slowly aggregating cluster where the motion due to 
gravitation is least in amount; and (2) the slight differential 
attraction away from the centre by the Milky Way on the side 
nearest to us. Again, this protective action of the Milky Way 
has been repeated, on a smaller scale, by the formation of the 
outer ring of the solar cluster, which has thus preserved the inner 
central cluster itself from a too abundant direct inflow of large 
masses of matter. 

But although the matter composing the outer portion of the 



304 MAN'S PLACE IN THE UNIVERSE 

original universe has been to a large extent aggregated into the 
vast system of the Milky Way, it seems probable, perhaps even 
certain, that some portion would escape its attractive forces and 
would pass through its numerous open spaces — indicated by the 
dark rifts, channels, and patches, as already described — and thus 
flow on unchecked towards the centre of mass of the whole system. 
The quantity of matter thus reaching the central cluster from 
the enormously remote spaces beyond the Milky Way might be 
very small in comparison with what was retained to build up that 
wonderful star-system; but it might yet be so large in total 
amount as to play an important part in the formation of the 
central group of suns. It would probably flow inwards almost 
continuously, and when it ultimately reached the solar cluster, 
it would have attained a very high velocity. If, therefore, it 
were widely diffused, and consisted of masses of small or mod- 
erate size as compared with planets or stars, it would furnish 
the energy requisite for bringing these slowly aggregating 
stars to the required intensity of heat for forming luminous 
suns. 

Here, then, I think, we have found an adequate explanation of 
the very long-continued light- and heat-emitting capacity of our 
sun, and probably of many others in about the same position in 
the solar cluster. These would at first gradually aggregate 
into considerable masses from the slowly moving diffused matter 
of the central portions of the original universe; but at a later 
period they would be reinforced by a constant and steady inrush 
of matter from its very outer regions, and therefore possessing 
such high velocities as materially to aid in producing and main- 



IMPORTANCE OE CENTRAL POSITION 305 

taining the requisite temperature of a sun such as ours, during 
the long periods demanded for continuous life-development. 
The enormous extension and mass of the original universe of 
diffused matter (as postulated by Lord Kelvin) is thus seen to 
be of the greatest importance as regards this ultimate product 
of evolution, because, without it the comparatively slow-moving 
and cool central regions might not have been able to produce and 
maintain the requisite energy in the form of heat ; while the ag- 
gregation of by far the larger portion of its matter in the great 
revolving ring of the Galaxy was equally important, in order to 
prevent the too great and too rapid inflow of matter to these 
favoured regions. 

It appears, then, that if we admit as probable some such proc- 
ess of development as I have here indicated, we can dimly see 
the bearing of all the great features of the stellar universe upon 
the successful development of life. These are, its vast dimen- 
sions ; the form it has acquired in the mighty ring of the Milky 
Way; and our position near to, but not exactly in, its centre. 
We know that the star-system has acquired these forms, pre- 
sumably from some simple and more diffused condition. We 
know that we are situated near the centre of this vast system. 
We know that our sun has emitted light and heat, almost uni- 
formly, for periods incompatible with rapid aggregation and the 
equally rapid cooling which physicists consider inevitable. I 
have here suggested a mode of development which would lead to 
a very slow but continuous growth of the more central suns ; to 
an excessively long period of nearly stationary heat-giving 
power ; and lastly, an equally long period of very gradual cooling 



306 MAN'S PLACE IN THE UNIVERSE 

— a period the commencement of which our sun may have just 

entered upon. 

Descending now to terrestrial physics, I have shown that, 
owing to the highly complex nature of the adjustments required 
to render a world habitable and to retain its habitability during 
the aeons of time requisite for life-development, it is in the high- 
est degree improbable that the required conditions and adap- 
tations should have occurred in any other planets of any other 
suns, which might occupy an equally favourable position as 
our own, and which were of the requisite size and heat-giving 
power. 

Lastly, I submit that the whole of the evidence I have here 
brought together leads to the conclusion that our earth is almost 
certainly the only inhabited planet in our solar system; and, 
further, that there is no inconceivability — no improbability even 
— in the conception that, in order to produce a world that should 
be precisely adapted in every detail for the orderly development 
of organic life culminating in man, such a vast and complex uni- 
verse as that which we know exists around us, may have been abso- 
lutely required. 

SUMMARY OF THE ARGUMENT 

As the last ten chapters of this volume embody a connected argu- 
ment leading to the conclusion above stated, it may be useful to 
my readers to summarise rather fully the successive steps of this 
argument, the facts on which it rests, and the various subsidiary 
conclusions arrived at. 

(1) One of the most important results of modern astronomy 



SUMMARY OF ARGUMENT 307 

is to have established the unity of the vast stellar universe which 
we see around us. This rests upon a great variety of observa- 
tions, which demonstrate the wonderful complexity in detail of 
the arrangement and distribution of stars and nebulae, combined 
with a no less remarkable general symmetry, indicating through- 
out a single inter-dependent system, not a number of totally dis- 
tinct systems so far apart as to have no physical relations with 
each other, as was once supposed. 

(2) This view is supported by numerous converging lines of 
evidence, all tending to show that the stars are not infinite in 
number, as was once generally believed, and which view is even 
now advocated by some astronomers. The very remarkable cal- 
culations of Lord Kelvin, referred to in the early part of this 
chapter, give a further support to this view, since they show that 
if the stars extended much beyond those we see or can obtain 
direct knowledge of, and with no very great change in their aver- 
age distance apart, then the force of gravitation towards the 
centre would have produced on the average more rapid motions 
than the stars generally possess. 

(3) An overwhelming consensus of opinion among the best 
astronomers establishes the fact of our nearly central position 
in the stellar universe. They all agree that the Milky Way is 
nearly circular in form. They all agree that our sun is situated 
almost exactly in its medial plane. They all agree that our sun, 
although not situated at the exact centre of the galactic circle, 
is yet not very far from it, because there are no unmistakable 
signs of our being nearer to it at any one point and farther 
away from the opposite point. Thus the nearly central posi- 



308 MAN'S PLACE IN THE UNIVERSE 

tion of our sun in the great star-system is almost universally 
admitted. 

On the question of the solar-cluster there is more difference of 
opinion; though here, again, all are agreed that there is such 
a cluster. Its size, form, density, and exact position are some- 
what uncertain, but I have, as far as possible, been guided by 
the best available evidence. If we adopt Lord Kelvin's general 
idea of the gradual condensation of an enormous diffused mass 
of matter towards its common centre of gravity, that centre would 
be approximately the centre of this cluster. Also, as gravita- 
tional force at and near this centre would be comparatively small, 
the motions produced there would be slow, and collisions being 
due only to differential motions, when they did occur would be 
very gentle. We might therefore expect many dark aggrega- 
tions of matter here, which may explain why we do not find any 
special crowding of visible stars in the direction of this centre; 
while, as no star has a sensible disc, the dark stars if at great 
distances would hardly ever be seen to occult the bright ones. 
Thus, it seems to me, the controlling force may be explained 
which has retained our sun in approximately the same orbit 
around the centre of gravity of this central cluster during the 
whole period of its existence as a sun and our existence as a 
planet ; and has thus saved us from the possibility — perhaps even 
the certainty — of disastrous collisions or disruptive approaches 
to which suns, in or near the Milky Way, and to a less extent 
elsewhere, are or have been exposed. It seems quite probable that 
in that region of more rapid and less controlled motions and 
more crowded masses of matter, no star can remain in a nearly 



SUMMARY OF ARGUMENT 309 

stable condition as regards temperature for sufficiently long 
periods to allow of a complete system of life-development on any 
planet it may possess. 

(4) The various proofs are next stated that assure us of the 
almost complete uniformity of matter, and of material physical 
and chemical laws, throughout our universe. This I believe no 
one seriously disputes ; and it is a point of the greatest impor- 
tance when we come to consider the conditions required for the 
development and maintenance of life, since it assures us that very 
similar, if not identical, conditions must prevail wherever organic 
life is or can be developed. 

( 5 ) This leads us on to the consideration of the essential char- 
acteristics of the living organism, consisting as it does of some 
of the most abundant and most widely distributed of these 
material elements, and being always subject to the general laws 
of matter. The best authorities in physiology are quoted, as to 
the extreme complexity of the chemical compounds which consti- 
tute the physical basis for the manifestation of life ; as to their 
great instability ; their wonderful mobility combined with per- 
manence of form and structure; and the altogether marvellous 
powers they possess of bringing about unique chemical transfor- 
mations and of building up the most complicated structures from 
simple elements. 

I have endeavoured to put the broad phenomena of vegetable 
and animal life in a way that will enable my readers to form some 
faint conception of the intricacy, the delicacy, and the mystery 
of the myriad living forms they see everywhere around them. 
Such a conception will enable them to realise how supremely 



310 MAN'S PLACE IN THE UNIVERSE 

grand is organic life, and to appreciate better, perhaps, the ab- 
solute necessity for the numerous, complex and delicate adapta- 
tions of inorganic nature, without which it is impossible for life 
either to exist now, or to have been developed during the im- 
measurable past. 

(6) The general conditions which are absolutely essential for 
life thus manifested on our planet are then discussed, such as, 
solar light and heat ; water universally distributed on the planet's 
surface and in the atmosphere; an atmosphere of sufficient 
density, and composed of the several gases from which alone 
protoplasm can be formed; some alternations of light and dark- 
ness, and a few others. 

(7) Having treated these conditions broadly, and explained 
why they are important and even indispensable for life, we next 
proceed to show how they are fulfilled upon the earth, and how 
numerous, how complex, and often how exact are the adjust- 
ments needed to bring them about, and maintain them almost 
unchanged throughout the vast aeons of time occupied in the 
development of life. Two chapters are devoted to this subject; 
and it is believed that they contain facts that will be new to 
many of my readers. The combinations of causes which lead to 
this result are so varied, and in several cases dependent on such 
exceptional peculiarities of physical constitution, that it seems 
in the highest degree improbable that they can all be found 
again combined either in the solar system or even in the stellar 
universe. It will be well here just to enumerate these conditions, 
which are all essential within more or less narrow limits : 

Distance of planet from the sun. 



SUMMARY OF ARGUMENT 311 

Mass of planet. 
Obliquity of its ecliptic. 
Amount of water as compared with land. 
Surface distribution of land and water. 
Permanence of this distribution, dependent probably on the 

unique origin of our moon. 
An atmosphere of sufficient density, and of suitable compo- 
nent gases. 
An adequate amount of dust in the atmosphere. 
Atmospheric electricity. 
Many of these act and react on each other, and lead to results 
of great complexity. 

(8) Passing on to other planets of the solar system, it is 
shown that none of them combine all the complex conditions 
which are found to work harmoniously together on the earth; 
while in most cases there is some one defect which alone removes 
them from the category of possible life-producing and life- 
supporting planets. Among these are the small size and mass 
of Mars, being such that it cannot retain aqueous vapour; and 
the fact that Venus rotates on its axis in the same time as it 
takes to revolve round the sun. Neither of these facts was 
known when Proctor wrote upon the question of the habitability 
of the planets. All the other planets are now given up — and 
were given up by Proctor himself — as possible life-bearers in 
their present stage; but he and others have held that, if not 
suitable now, they may have been the scene of life-development 
in the past, while others will be so in the future. 

In order to show the futility of this supposition, the problem 



312 MAN'S PLACE IN THE UNIVERSE 

of the duration of the sun as a stable heat-giver is discussed; 
and it is shown that it is only by reducing the periods claimed 
by geologists and biologists for life-development upon the earth, 
and by extending the time allowed by physicists to its utmost 
limits, that the two claims can be harmonised. It follows that 
the whole period of the sun's duration as a light- and heat-giver 
has been required for the development of life upon the earth; 
and that it is only upon planets whose phases of development 
synchronise with that of the earth that the evolution of life is 
possible. For those whose material evolution has gone on quicker 
or slower, there has not been, or will not be, time enough for the 
development of life. 

( 9 ) The problem of the stars as possibly having life-support- 
ing planets is next dealt with, and reasons are given why in only 
a minute portion of the whole is this possible. Even in that 
minute portion, reduced probably to a few of the component suns 
of the solar cluster, a large proportion seems likely to be ruled 
out by being close binary systems, and another large portion by 
being in process of aggregation. In those remaining, whether 
they may be reckoned by tens or by hundreds we cannot say, the 
chances against the same complex combination of conditions as 
those which we find on the earth occurring on any planet of any 
other sun are enormously great. 

(10) I then refer, briefly, to some recent measurements of 
star-radiation, and suggest that they may thus possibly have 
important effects on the development of vegetable and animal 
life; and, finally, I discuss the problem of the stability of the 
stellar universe and the special advantage we derive from our 



CONCLUSION 313 

central position, suggested by some of the latest researches of 
our great mathematician and physicist — Lord Kelvin. 

CONCLUSIONS 

Having thus brought together the whole of the available evidence 
bearing upon the questions treated in this volume, I claim that 
certain definite conclusions have been reached and proved, and 
that certain other conclusions have enormous probabilities in their 
favour. 

The conclusions reached by modern astronomers are: 

( 1 ) That the stellar universe forms one connected whole ; and, 
though of enormous extent, is yet finite, and its extent deter- 
minable. 

( 2 ) That the solar system is situated in the plane of the Milky 
Way, and not far removed from the centre of that plane. The 
earth is therefore nearly in the centre of the stellar universe. 

( 3 ) That this universe consists throughout of the same kinds of 
matter, and is subjected to the same physical and chemical laws. 

The conclusions which I claim to have shown to have enor- 
mous probabilities in their favour are — 

(4) That no other planet in the solar system than our earth 
is inhabited or habitable. 

(5) That the probabilities are almost as great against any 
other sun possessing inhabited planets. 

(6) That the nearly central position of our sun is probably 
a permanent one, and has been specially favourable, perhaps ab- 
solutely essential, to life-development on the earth. 



314 MAN'S PLACE IN THE UNIVERSE 

These latter conclusions depend upon the combination of a 
large number of special conditions, each of which must be in 
definite relation to many of the others, and must all have per- 
sisted simultaneously during enormous periods of time. The 
weight to be given to this kind of reasoning depends upon a full 
and fair consideration of the whole evidence as I have en- 
deavoured to present it in the last seven chapters of this book. 
To this evidence I appeal. 

This completes my work as a connected argument, founded 
wholly on the facts and principles accumulated by modern 
science ; and it leads, if my facts are substantially correct and my 
reasoning sound, to one great and definite conclusion — that man, 
the culmination of conscious organic life, has been developed here 
only in the whole vast material universe we see around us. I 
claim that this is the logical outcome of the evidence, if we con- 
sider and weigh this evidence without any prepossessions what- 
ever. I maintain that it is a question as to which we have no 
right to form a priori opinions not founded upon evidence. And 
evidence opposed to this conclusion, or even as to its improb- 
ability, we have absolutely none whatever. 

But, if we admit the conclusion, nothing that need alarm either 
the scientific or the religious mind necessarily follows, because it 
can be explained or accounted for in either of two distinct ways. 
One considerable body, including probably the majority of men 
of science, will admit that the evidence does apparently lead to 
this conclusion, but will explain it as due to a fortunate coin- 
cidence. There might have been a hundred or a thousand life- 



CONCLUSION 315 

bearing planets, had the course of evolution of the universe been 
a little different, or there might have been none at all. They 
would probably add, that, as life and man have been produced, 
that shows that their production was possible; and therefore, 
if not now then at some other time, if not here then in some other 
planet of some other sun, we should be sure to have come into 
existence ; or if not precisely the same as we are, then something 
a little better or a little worse. 

The other body, and probably much the larger, would be 
represented by those who, holding that mind is essentially 
superior to matter and distinct from it, cannot believe that life, 
consciousness, mind, are products of matter. They hold that the 
marvellous complexity of forces which appear to control matter, 
if not actually to constitute it, are and must be mind-products ; 
and when they see life and mind apparently rising out of matter 
and giving to its myriad forms an added complexity and un- 
fathomable mystery, they see in this development an additional 
proof of the supremacy of mind. Such persons would be inclined 
to the belief of the great eighteenth-century scholar, Dr. Bent- 
ley, that the soul of one virtuous man is of greater worth and ex- 
cellency than the sun and all his planets and all the stars in the 
heavens ; and when they are shown that there are strong reasons 
for thinking that man is the unique and supreme product of this 
vast universe, they will see no difficulty in going a little further, 
and believing that the universe was actually brought into exist- 
ence for this very purpose. 

With infinite space around us and infinite time before and 
behind us, there is no incongruity in this conception. A universe 



316 MAN'S PLACE IN THE UNIVERSE 

as large as ours for the purpose of bringing into existence many 
myriads of living, intellectual, moral, and spiritual beings, with 
unlimited possibilities of life and happiness, is surely not more 
out of proportion than is the complex machinery, the lifelong 
labour, the ingenuity and invention which we have bestowed upon 
the production of the humble, the trivial, pin. Neither is the 
apparent waste of energy so great in such a universe, compara- 
tively, as the millions of acorns, produced during its life by an 
oak, every one of which might grow to be a tree, but of which 
only one does actually, after several hundred years, produce the 
one tree which is to replace the parent. And if it is said that the 
acorns are food for bird and beast, yet the spores of ferns and 
the seeds of orchids are not so, and countless millions of these 
go to waste for every one which reproduces the parent form. 
And all through the animal world, especially among the lower 
types, the same thing is seen. For the great majority of these 
entities we can see no use whatever, either of the enormous variety 
of the species, or the vast hordes of individuals. Of beetles alone 
there are at least a hundred thousand distinct species now living, 
while in some parts of sub-arctic America mosquitoes are some- 
times so excessively abundant that they obscure the sun. And 
when we think of the myriads that have existed through the 
vast ages of geological time, the mind reels under the immensity 
of, to us, apparently useless life. 

All nature tells us the same strange, mysterious story, of the 
exuberance of life, of endless variety, of unimaginable quantity. 
All this life upon our earth has led up to and culminated in that 
of man. It has been, I believe, a common and not unpopular 



CONCLUSION 317 

idea that during the whole process of the rise and growth and 
extinction of past forms, the earth has been preparing for the 
ultimate — Man. Much of the wealth and luxuriance of living 
things, the infinite variety of form and structure, the exquisite 
grace and beauty in bird and insect, in foliage and flower, may 
have been mere by-products of the grand mechanism we call 
nature — the one and only method of developing humanity. 

And is it not in perfect harmony with this grandeur of design 
(if it be design), this vastness of scale, this marvellous process 
of development through all the ages, that the material universe 
needed to produce this cradle of organic life, and of a being- 
destined to a higher and a permanent existence, should be on a 
corresponding scale of vastness, of complexity, of beauty? 
Even if there were no such evidence as I have here adduced for the 
unique position and the exceptional characteristics which dis- 
tinguish the earth, the old idea that all the planets were in- 
habited, and that all the stars existed for the sake of other 
planets, which planets existed to develop life, would, in the light 
of our present knowledge, seem utterly improbable and incredible. 
It would introduce monotony into a universe whose grand char- 
acter and teaching is endless diversity. It would imply that to 
produce the living soul in the marvellous and glorious body of 
man — man with his faculties, his aspirations, his powers for good 
and evil — that this was an easy matter which could be brought 
about anywhere, in any world. It would imply that man is an 
animal and nothing more, is of no importance in the universe, 
needed no great preparations for his advent, only, perhaps, a 
second-rate demon, and a third or fourth-rate earth. Looking 



318 MAN'S PLACE IN THE UNIVERSE 

at the long and slow and complex growth of nature that pre- 
ceded his appearance, the immensity of the stellar universe with 
its thousand million suns, and the vast aeons of time during which 
it has been developing — all these seem only the appropriate and 
harmonious surroundings, the necessary supply of material, the 
sufficiently spacious workshop for the production of that planet 
which was to produce, first, the organic world, and then, Man. 

In one of his finest passages our great world-poet gives us his 
conception of the grandeur of human nature — " What a piece 
of work is man ! How noble in reason ! How infinite in faculty ! 
In form and moving, how express and admirable! In action 
how like an angel ! In apprehension how like a god ! " And for 
the development of such a being what is a universe such as ours ? 
However vast it may seem to our faculties, it is as a mere nothing 
in the ocean of the infinite. In infinite space there may be infinite 
universes, but I hardly think they would be all universes of 
matter. That would indeed be a low conception of infinite 
power! Here, on earth, we see millions of distinct species of 
animals, millions of different species of plants, and each and 
every species consisting often of many millions of individuals, 
no two individuals exactly alike ; and when we turn to the heavens, 
no two planets, no two satellites alike ; and outside our system we 
see the same law prevailing — no two stars, no two clusters, no 
two nebulae alike. Why then should there be other universes of 
the same matter and subject to the same laws — as is implied by 
the conception that the stars are infinite in number, and extend 
through infinite space? 

Of course there may be, and probably are, other universes, 



CONCLUSION 319 

perhaps of other kinds of matter and subject to other laws, per- 
haps more like our conceptions of the ether, perhaps wholly 
non-material, and what we can only conceive of as spiritual. 
But, unless these universes, even though each of them were a 
million times vaster than our stellar universe, were also infinite 
in number, they could not fill infinite space, which would extend 
on all sides beyond them, so that even a million million such uni- 
verses would shrink to imperceptibility when compared with the 
vast beyond! 

Of infinity in any of its aspects we can really know nothing, 
but that it exists and is inconceivable. It is a thought that 
oppresses and overwhelms. Yet many speak of it glibly as if 
they knew what it contains, and even use that assumed knowledge 
as an argument against views that are unacceptable to them- 
selves. To me its existence is absolute but unthinkable — that 
way madness lies. 

" O night! O stars, too rudely jars 
The finite with the infinite ! " 

I will conclude with one of the finest passages relating to the 
infinite that I am acquainted with, from the pen of the late 
R. A. Proctor: 

" Inconceivable, doubtless, are these infinities of time and 
space, of matter, of motion, and of life. Inconceivable that the 
whole universe can be for all time the scene of the operation of 
infinite power, omnipresent, all-knowing. Utterly incomprehen- 
sible how Infinite Purpose can be associated with endless material 
evolution. But it is no new thought, no modern discovery, that 



320 MAN'S PLACE IN THE UNIVERSE 

we are thus utterly powerless to conceive or comprehend the idea 
of an Infinite Being, Almighty, All-knowing, Omnipresent, and 
Eternal, of whose inscrutable purpose the material universe is the 
unexplained manifestation. Science is in presence of the old, 
old mystery ; the old, old questions are asked of her — ' Canst 
thou by searching find out God? Canst thou find out the Al- 
mighty unto perfection? It is as high as heaven; what canst 
thou do? deeper than hell ; what canst thou know? ' And science 
answers these questions as they were answered of old — ' As 
touching the Almighty we cannot find Him out.' " 

The following beautiful lines — among the latest products of 
Tennyson's genius — so completely harmonise with the subject- 
matter of the present volume, that no apology is needed for quot- 
ing them here. 

(The Question) 

Will my tiny spark of being 

Wholly vanish in your deeps and heights? 

Must my day be dark by reason, 

O ye Heavens, of your boundless nights, 

Bush of Suns and roll of systems, 
And your fiery clash of meteorites? 

(The Answer) 

"Spirit, nearing yon dark portal 

At the limit of thy human state, 
Fear not thou the hidden purpose 

Of that Power which alone is great, 
Nor the myriad world, His shadow, 

Nor the silent Opener of the Gate." 




IIX 







w^ 




NF.BUL.E 

RF.SOLV ABIE NEBULA , 
CLDSTF.BS 



THE NEBUL/E AND CLUSTERS OF THE SOUTHERN HEAVENS. 

Drawn upon an equal surface projection from Dr DREYER'S Catalogue of 
The Milky Way from The URANOM ETRIA ARGENTINA. 
By SIDNEY WATERS. 



#« 




THE NEBUL/E AND CLUSTERS OF THE NORTHERN HEAVENS. 
Mwa .,,...„ ... ».,u«l ...rf..-. projntloil IWun !>r DRBYER-S 

"'■ "■■'-> « 4 ' Dl B Kl i; S It, wing. 

By SIDNEY WATERS. 



INDEX 



Adrian us Tollius on stone axes, 
201. 

Air criminally poisoned by us, 256. 

Albedo explained, 162. 

Algol and its companion, 40; 
change of colour of, 42. 

Allen, Prof. F. J., on living matter, 
192; on importance of nitrogen, 
194; on physical conditions essen- 
tial for life, 195. 

Alpha Centauri, nearest star, 74. 

Ammonia, importance of, to life, 
194. 

Anaximander's cosmic theory, 4. 

Angles of a minute and second, 80. 

Arcturus, rapid motion of, 172. 

Argument of book, summary of, 
306. 

Astronomers, the first, 4. 

Astronomy, the new, 25. 

Astrophysics, a new science, 33- 

Atmosphere, qualities requisite for 
life, 209; requisite composition 
of, 211; aqueous vapour in, 212; 
and life, 240; effects of density 
of, 241; a complex structure, 255; 
its vital importance to us, 258. 

Ball, Sir R., on dark stars, 142; 

Time and Tide, 231. 
Barnham, S. W., on double stars, 



Blue of sky due to dust, 247. 
Boeddicker's map of Milky Way, 

164. 
Brewster, Sir D., against Whewell, 

16. 

Campbell, Prof., on spectroscopic 
binaries, 124; on uncertainty of 
sun's motions, 178; on number of 
binary systems, 282. 

Carbon-compounds, vast numbers 
of, 193. 

Carbonic acid gas essential for life, 
195. 

Central position of sun, importance 
of, 301. 

Chaldeans the first astronomers, 4. 

Chalmers, Dr., on plurality of 
worlds, 15. 

Chamberlin, T. C, origin of nebulae, 
120; on stellar disruption, 185. 

Chromosphere, the sun's, 107. 

Clerke, Miss A. M., on limits of 
star system, 138; on Milky Way, 
158, 160; on solar cluster, 165; 
on uncertainty of the sun's mo- 
tion, 176. 

Climate, persistence of mild, 220. 

Clouds, importance of, to life, 244. 

Clusters in relation to Galaxy, 67. 

Comte, on impossibility of real 
knowledge of the stars, 26. 



321 



322 



INDEX 



Conclusions of the book, 313; bear- 
ing of, on science and on religion, 
314. 

Corona of sun, 108. 

Criticisms of article in Fortnightly 
Review, 168, 179. 

Darwin, Prof. G., on meteoritic 
hypothesis, 133; on origin of 
moon, 230; on instability of an- 
nular systems, 291. 

Day and night, uses of, 213. 

Diagrams of star-distribution, 62, 
65. 

Diffraction-gratings, 31. 

Disruption of stellar bodies, 185. 

Doppler principle, the, 37. 

Double stars, evolution of, 122; not 
fitted for life, 282. 

Dust, importance of, 245. 

Dust-free air, results of, 250. 

Earth, first measured, 6; in rela- 
tion to life, 216; the only habi- 
table planet, 258; cannot retain 
hydrogen, 260; supposed extreme 
conditions of, 267. 

Earth's mass, how related to life, 
261. 

Ecliptic, obliquity of, in relation to 
life, 217. 

Electricity, effects of atmospheric, 
254; atmospheric, how caused, 
254. 

Elements, change in spectra of, 
128; in the sun, 183; in meteor- 
ites, 184; in organic structures, 
199. 

Empedocles an early astronomer, 5. 



Eudoxus on motions of planets, 5. 
Evolution of the stars, 128. 
Explanations of life-processes, 200. 

Facul;e of sun, 105. 

Fisher, Rev. O., on oceanic basins, 

232; on thin sub-oceanic crust, 

235. 

Fizeau measures speed of light, 78. 

Flammarion, C, on universality of 

life, 270, 276. 
Fontanelle on plurality of worlds, 11. 

Galileo on star measurement, 74. 
Geological climates, 220. 
Geologists on duration of sun's 

heat, 271. 
Germinal vesicle, M'Kendrick on, 

201. 
Gill, Sir D., on systematic star- 
motions, 178. 
Globular clusters, stability of, 125; 

and variables, 127. 
Gore, Mr. J. E., on stars in Galaxy, 

60; on mass of binary stars, 96; 

on remoteness of bright stars, 

140 ; on limits of star-system, 145 ; 

on limited number of stars, 150; 

on life on planets of other suns, 

278, 285. 
Gould on solar cluster, 165. 
Gould's map of Milky Way, 164. 
Gravitation, motions produced by, 

on Lord Kelvin's hypothesis, 293. 

Haliburtok, Professor W. D., on 

proteids, 198. 
Hall, Mr. Maxwell, on star-motions, 

178. 



INDEX 



323 



Heat and cold on earth's surface, 

206. 
Heat-supply, our long-continued, 

accounted for, 301. 
Herschel, Sir J., on Milky Way, 

50; on limits of the star-system, 

147. 
Heliometer, description of, 88. 
Huggins, Sir W., on spectra of 

stars, 33; measures radial mo- 
tion, 38. 
Huxley, Prof., on protoplasm, 197; 

on duration of life, 274. 
Hydrogen, why not in atmosphere, 

237; escapes from earth, 260. 

i 
Infinity, unknowable, 319; Proctor 

on, 319. 



on temperature, 216; now im- 
probable in stars, 284; conditions 
essential for, summarised, 310. 

Life-processes, explanations of, 201. 

Light, velocity of, measured, 78; 
necessity of solar, 208; from sky 
due to dust, 249. 

Light-journey explained, 75. 

Light-ratio shows stars to be lim- 
ited, 151. 

Living bodies, essential points in, 
191. 

Lockyer, Sir N., on inorganic evo- 
lution, 116; on evolution of stars, 
130; on Milky Way, 160; on posi- 
tion of solar system, 161. 

Luigi d'Auria on stellar motion, 
302. 



Jupiter's satellites show speed of 
light, 79. 

Kapteyn on solar cluster, 166. 

Kelvin, Lord, on the sun's age, 274; 
on a suggested primitive form of 
star-system, 294. 

Kirchhoff, discovers spectrum-an- 
alysis, 29. 

Laws of matter uniform through- 
out universe, 186. 

Leaves, importance of, 196. 

Lee, Dr., on origin of double stars, 
123. 

Lewis, on remote bright stars, 141. 

Life, unity of organic, 188; defini- 
tions of, 190; conditions essential 
for, 205; water essential for, 208; 
atmosphere for, 209; dependent 



M'Kendrick, Prof., on germinal 

vesicle, 201. 
Magnetism and sun-spots, 106. 
Man, Shakespeare on, 318. 
Mars, has no water, 262; excessive 

temperatures on, 263. 
Matter of universe uniform, 182. 
Maunder on dark stars, 142. 
Measurement of star-distances, 86; 

difficulty of, 86. 
Mercury not habitable, 262. 
Meteorites, elements in, 184; not 

primitive bodies, 185. 
Meteoritic hypothesis, 113; Proctor 

on, 114; explains nebulae, 116; 

Dr. Roberts on, 118. 
Milky Way, the, 48; form of, 50, 

159; description of, 52; telescopic 

view of, 57; stars in relation to, 

59; Mr. Gore on, 60; density of 



324 



INDEX 



stars in, 61 ; clusters and nebulae 
in relation to, 67; probable dis- 
tance of, 95; forms a great circle, 
157, 162; Professor Newcomb on, 
158; probably no life in, 280; dia- 
gram of, 296; revolution of, im- 
portant to us, 303. 

Million, how to appreciate a, 82. 

Minchin, G. M., on radiation from 
stars, 286. 

Monck, Mr. W. H. S., on non-in- 
finity of stars, 144; on uncer- 
tainty of sun's motion, 177. 

Moon, why no atmosphere, 259. 

Moon's supposed origin, 230. 

Motion, in line of sight, 36. 

Motions, imperceptible, 39. 

Nebula, with gaseous spectra, 43; 
in relation to Galaxy, 66; distri- 
bution of, 68; many forms of, 70; 
gaseous, 71; meteoritic theory of, 
116; planetary and annular, 174, 
175; Dr. Roberts on spiral, 117, 
173; Chamberlin on origin of, 
120. 

Nebular hypothesis, 97, 111; objec- 
tion to, 112. 

Newcomb, Prof. S., on star distri- 
bution, 61; on parallax of stars, 
94; on stability of star clusters, 
126; on scarcity of single stars, 
128; on limits of star-system, 
138; on Milky Way, 158, 160; on 
solar cluster, 167; star velocities, 
171; on average small mass of 
stars, 280; on star-motions, 293. 

Newton, Sir Isaac, on sun's habi- 
tability, 11. 



Nichols, E. F., on heat of stars, 

286. 
Nitrogen, its importance to life, 

194. 
Non-habitability of great planets, 



Ocean and land, diagram of, 226. 

basins, permanence of, 226. 

symmetry of, 236. 

depths, how produced, 229. 

Oceans, effect of, on temperature, 

236; curious relations of, 260. 
Organic products, diversity of, 193, 

194. 

Photographic astronomy, 43; meas- 
ures of star-distances, 89. 

Photosphere, the, 105. 

Physicists on sun's duration, 274. 

Pickering's measurements of Algol, 
40. 

Planets, supposed habitability of, 
262, 265; the great, uninhabitable, 
268; internal heat of great, 268; 
a last argument for habitability 
of, 270; have probably no life, 
311. 

Planets' motions first explained, 5; 
mass and atmosphere, 260. 

Pleiades, number of stars in, 67; 
a drifting cluster, 176. 

Plurality of worlds, early writers 
on, 11; Proctor on, 19. 

Posidonius measures the earth, 7. 

Pritchard's photographic measures 
of star-distance, 89. 

Proctor, R. A., on other worlds, 19; 
on form of Galaxy, 51; on Her- 



INDEX 

schel's views, 101 ; on stellar uni- 
verse, 103; on meteor itic theory, 
114; on infinities, 136; on star- 
drift, 176; on life under varied 
conditions, 267; on infinity, 319. 

Proctor's Old and New Astronomy , 
46; chart of stars, 60. 

Prominences of sun, 107. 

Proteids, formation of, 198; Prof. 
Haliburton on, 198. 

Protoplasm, complexity of, 193; a 
mechanism, 197; sensibility of, to 
heat, 207. 

Ptolemaic system of the heav- 
ens, 6. 



325 



Radial motion, 36. 

Radiation from stars, 285. 

Rain in the Carboniferous age, 223; 
dependent on dust, 245. 

Ramsay, Prof., on geological cli- 
mates, 273. 

Ranyard, on star-discs, 97; on in- 
finite universe, 137; on mass of 
Orion nebula, 172. 

Religious bearing of my conclu- 
sions, 314. 

Reproduction, marvel of, 200. 

Reversing layer of sun, 106. 

Roberts, A. W., on birth of double 
stars, 123. 

Dr. I., on limits of star-system, 

148; on spiral nebulae, 117; on 
meteoritic theory, 118; photo- 
graphs of nebulae, 45, 173. 

Roche limit explained, 120, 185. 

Sanderson, Prof. Burdon, on living 
matter, 191. 



Scientific and agnostic opinion on 
my conclusions, 314. 

Secchi's classification of stars, 34. 

Single stars perhaps rare, 128. 

Solar apex, position of, 175. 

Solar cluster, the, 165; diagram 
showing, 296; evidence for, 298; 
importance to us, 302-3, 308. 

Solar system, position of, 300. 

Sorby on constitution of meteorites, 
185. 

Spectra, varieties of, 34; of ele- 
ments, changes in, 128. 

Spectroscopic binaries, abundance 
of, 124; great numbers of, 282. 

Spectrum-analysis, discovery of, 
27. 

Spencer, H., on status of nebulae, 
102. 

Spiral nebulae, origin of, 120. 

Stars, proved to be suns, 33; in- 
visible, 39; classification of, 33; 
spectroscopic double, 42; distri- 
bution of the, 47; number of 
visible, 47; description of Milky 
Way, 52; in relation to Milky 
Way, 59; distances of, 74; meas- 
urement of distance of, 86; mass 
of binary, 96; evolution of double, 
122; spectroscopic double, 122; 
clusters of, 125; evolution of 
the, 128; classification of, 129, 130; 
the hottest, 131; when cooling 
give more heat, 131; cycle of evo- 
lution and decay, 132; supposed 
Lifinite number of, 135; not in- 
finite, 137; law of diminishing 
numbers of, 149; systematic mo- 
tions of, 177; in relation to life, 



326 



INDEX 



.0 



278, 282; possible use of their 

emanations, 285. 
Star-clusters and variables, 127. 
Star-density, diagram of, 65. 
Star-drift, Proctor on, 176. 
Star-motions, Prof. Newcomb on, 

293. 
Starlight, electrical measure of, 

286; possible uses of, 288. 
Star-system, limited, 145; stability 

of, 291; supposed primitive form 

of, 293. 
Stellar motion, Luigi d'Auria on, 

302. 

universe, shape of, 49; unity 

of, 100; evolution of, 103; dia- 
grams of, 296, 297. 

Stoney, Dr., on atmospheres and 

gravity, 259. 
Sun a typical star, 104; brightness 

of, 104; heat of, 104; surface of, 

105; surroundings of, 106-110; 

corona of, 108; colour of, 110; 

elements in, 183. 
Sun-spots, nature of, 105. 
Sun's distance, measure of, 76. 

life, all required to develop 

earth-life, 275. 

motion through space, 91, 169. 

uncertain, 177. 

heat, supposed limits of, 271. 

Symmetry of oceans, cause of, 235. 



Temperature, essential for lift,. 
205; equalised by water, 236; as 
regards life on planets, 262. 

Tennyson on man and the uni- 
verse, 320. 

Uniformity of matter, 182. 
Unity of stellar universe, 100. 
Universe of stars, how its form 

has affected our sun and earth, 

304. 
Universe not disproportionate if 

man is its sole product, 316. 

Vexus, radial motions of, 38; dia- 
gram of transit of, 77; life barely 
possible on, 262; adverse climatic 
conditions of, 264. 

Water, an essential for life, 208; 

its amount and distribution, 225; 

an equaliser of temperature, 236. 
Wave-lengths, how measured, 32. 
Whewell, on plurality of worlds, 10, 

16; on man as the highest product 

of the universe, 16. 
Whittaker, Mr. E. T., on gravita- 

tive and electro-dynamical forces, 

292. 
Winds, importance of, to life. 243. 

Zodiacal light, 109. 



THE END 



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